I think you took my comparison too concretely.

Communism is about as similar to the GPL as it is to pro-lifers. Nothing more, and I did not mean to imply any more similarity than that.

To help you grab on to my extended metaphor:

Communism: a communist controlled authoritarian country => GPL: the subset of _released_ software licensed under the GPL

Communism: your natural free will and ability to make a choice => GPL: Your guaranteed rights (in free world) to intellectual property

Communism: everyone's forced to be selfless (fair redistribution of wealth)... under penalty of death? => GPL: everyone's forced to license under GPL under penalty of being sued

Communism: if not forced, one person could shift the economy because communism != economic equilibrium => GPL: If GPL does not force GPL on derivatives, then one proprietary software could shift the balance of power so that it's more advantageous not to open source. This is because if on guy doesn't share, he expends less resources on helping others, while taking in fruits of others' labors.

Public domain is like a family, or really small communal group, whereas GPL is like some authoritarian communist state. You've got the family which works well on the ideal of dedication to the whole. It's small, but it's all by free choice. You've got communist China supposedly trying to force this ideal of a harmonious multi-billion member family on its people, while probably never going to achieve it. Sure they're converting more people to communism, but at what cost?

There is no absolute force. There is always at least 2 distinct choices, and any "forcing" as we know it is a result of external pressure. The GPL exerts an external pressure on decisions, so for all intents and purposes, it forces, thus limiting true freedom. True freedom is when there are no external pressures on a decision.

Communism (in practice): In order to propagate and preserve itself it needs to be forced upon people because the slightest selfish act topples the whole system. This is opposed to letting everyone choose to be selfless (not bloody likely, which is why force is necessary).
GPL: In order to propagate and preserve itself it needs to enforce its doctrine by law whether or not it makes practical sense in individual cases. This is opposed to letting everyone choose when it's appropriate and practical to open source, and when to proprieties (like in Communism, it can be disadvantageous for all if only a few keep their software proprietary). However I do concede that since the GPL does not force you to release the changes it is one good thing about it.
Pro-lifers (in practice): In order to propagate life and preserve the sanctity of it, it is necessary to force people to do the morally right thing, whether or not it's practical in each individual case.

Common themes: absolute black and white world view; end justifying means; restricting free choice
It's particularly hypocritical of the GPL, which espouses freedom, to achieve it's goal through restriction of freedom.
Hey sure if we are all nice selfless people we could live in a global commune. If we are all righteous, we wouldn't need to be forced not to kill (or make) babies we don't want. And if we all understood the benefits of free software, we wouldn't need to be forced. But we aren't Jesus, aren't righteous, and aren't smart enough to appreciate free software. Therefore it's unfair to encourage anything that forces people to pretend the world is ideal.

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>Now, can you name one ideal which is compatible with our world? Even slavery isn't completely abolished. I mean, any political idea will have its opponents.

Your question is an oxymoron. By definition anything ideal is unattainable, and as for the purposes of our discussion, it means it's also not compatible with our world. But I don't know why you are treating me like I'm Microsoft, I love free software. I've learned all I know from free software and it's let me build some wonderful things. But I love free software for freedom. And the GPL is not free. It's trying to make the world free, but by utilizing the very restriction on software freedoms it's trying to abolish. Hey if free software is really good for everyone, why not let it propagate by its own merits, rather than artificial restrictions/terms? I for one have faith in the practical benefits of free software, and don't believe it's necessary to force (if you want to benefit from GPL) people to adopt it.

> If your conclusion is that a software licence shouldn't embody political ideas, take a look at proprietary software licences.
Not my conclusion at all, I think free-er licenses are just as political.

> Restrictions like "thou shalt not study this software" are indeed quite political.
And what's the difference between "thou shalt not study this software" and "thou must make any changes public upon distribution"? They both restrict your rights. And the only time the GPL's restriction wouldn't matter is if the whole world's softwares were under GPL. But that ain't happening any time soon, and sure as hell ain't going to happen if I'm forced to do it.

> It was specifically designed with our current world in mind, to make it go from proprietary software to free software, progressively.
Yes progressively by force... by constantly limiting our choices until one day we've got none but GPL (or do everything yourself).

>Remember: the political agenda behind the GPL is freedom.
The whole idea of freedom is to have freedom in means, not just the ends. Plus, being an ideal, that end will never be achieved, which means all we've got are the means. So if the means aren't free, we'll never be free. So given that the GPL restricts our rights to try to achieve a probably unattainable ideal, we'll probably be stuck having our rights restricted until the end of world.

> You can't, however, oppose the GPL without being in the camp of proprietary software, at least to some extent.
That's like saying you can't be pro-choice without being at least a little pro-death (which is not true btw). Like I said before, I want all software to be free, but I'm not going to waste my time restricting rights while trying to achieve that goal.