There should be something done to change the attitudes of plugin/theme developers. As we see from the comments on previous article

I agree that the people behind Formidable Forms should not be blamed for the buyer’s behaviour, and shouldn’t apologize or anything.

They still think that they can do whatever they want with their code and they even "shouldn’t apologize or anything"... That's right, legally they can, but being legal doesn't make it necessary right (slavery for example) and it doesn't make this behavior ethical.

Is it so hard to understand, that you don't sell your code - if anyone wants you code, they can just fork it. Yes, you are selling the "name" and reputation, but must of all you're selling access to people's sites. And if you sell it to a highest (or perhaps the only) bidder, without properly checking, you act irresponsibly.

Did you do all checks, were you 100% sure, you give the plugin into good hands? If not, then I'm 100% happy that all plugins of such irresponsible developer would be closed by admins. No need to cry, your former users are crying now...