I see what you are trying to say and the issue of lack of European fiscal solidarity is real, but there's a lot of big whoppers in your post, so you may want to argue somewhat differently. Let's go through them:

> California and Alabama are in a monetary and customs union AND in a fiscal a political union!

Kind of. They are both subordinate states to the Federal government which taxes and spends far more than any state, so that third actor -- the Federal government - is the gorilla in the room, and needs to be included.

> Federal money flow is net positive into Alabama and net negative out of California

No, both California and Alabama are famously receiving states. California used to be a donor state in the past - Governor Schwarzenegger famously complained about getting back 70 cents for every dollar, but that number was rapidly increasing even in his tenure and about 5 years ago it exceeded 100 and it still continues to rapidly rise. So now both California and Alabama are firmly in the receiver bucket. In fact, all states can be in the receiver bucket with the Federal government running large deficits.

> There's no tax havens inside the US for states to steal tax revenue from one another.

This is exactly what the SALT deduction is about as well as tax-free state and local bonds. One of the reasons why states borrow so much is because you do not pay federal income taxes on the interest (in most cases).

https://investor.vanguard.com/investing/taxes/government-bon...

But to understand that you need to know about state borrowing, which takes us to the next point:

> Can you imagine if each state had to emit it's own bonds, lol!

States and local governments emit quite a lot of bonds. California owes about 70 Billion in bonds outstanding, but that doesn't even count things like "capital appreciation bonds" and other types of instruments it sells. See here:

https://www.treasurer.ca.gov/cdiac/debtdata/debtdata.asp

https://www.treasurer.ca.gov/cdiac/debtdata/totals.asp

Even individual cities and counties sell bonds, as do port authorities, etc.

> Imagine each state being on its own, and Alabama trying to get finance by selling Alabama t-bills on Alabama credit?

Yes, we showed the bonds that California sells and Alabama also sells bonds. Each state has debt, it's various bonds are given a credit rating, etc. FYI Alabama's latest bonds have a credit rating (AA+) that is slightly higher than California's latest issuance (AA), because Alabama's finances are in better shape.

https://www.fitchratings.com/entity/alabama-state-of-al-2356

https://www.fitchratings.com/entity/california-state-of-ca-2...

So I think you should revisit your argument because the situation is a lot more complex. It's better to think of the federal government as providing income insurance to the states, rather than just pretending that states do not independently borrow money, that they do not compete with each other, and that they do not have their own credit ratings. A better analogy is to look at individuals in a nation who each borrow, save, and spend, but they are protected with some federal programs like welfare and disability insurance. That's a much better way to view the situation.