I know, you're not supposed to lead with a negative. Here's the reasoning. If you do it the wrong way first, you'll get a reputation as a spammer, and that will kill any chance you have in most groups. There are people who keep tabs on this sort of stuff, and if they spread the word that you got nabbed advertising in the wrong places, it can be a problem. Usually it takes more than one incident before it gets to be trouble, though.
People make mistakes, and that's taken into account.
In the ones where it is, advertising is usually ineffective. In the groups where advertising is allowed, the vast majority of people who will see your ad will be those who want to sell you something, not folks who are looking for services or products to buy. The usefulness of most advertising groups was killed long ago by people who insist on posting off topic ads for products that the group was not intended to cover, and selling things that most people wouldn't buy even if they were presented professionally. Like walking into a private gathering with a bullhorn and shouting out a sales pitch, then leaving. If they let your ISP know, there's a good chance you'll lose your account after the second time.