Although I'm sympathetic to the original developer, I think it is worth calling a spade a spade: this was a passive aggressive way to get the plugin removed. Rather than hiding behind the Readme, he should have just done what Joost does.

That said, I also think that this situation is an important reminder that the GPL works both ways. Removing/replacing all references to WP Migrate Pro -- even at the code level (and see above comment, I'm not actually sure that's required, if follow whatever CentOS and Scientific Linux do), is a simple regular expression or two. A DMCA can't change that.

Part of releasing under the GPL means having to deal with stuff like this. For good and for bad. I'm sympathetic to a point, but if you choose to base your business on something that is licensed on the GPL, you better be prepared for this to happen.

The only real way to pseudo-avoid stuff like this is to make part of the plugin a server-side component -- and I don't just mean a validation check, I mean some of the code that is required for the plugin to run would have to exist on a server. That wouldn't really work well in this case - since local databases are part of the target audience. More to the point, if you spend so much time obfuscating your code so that others can't "steal" it, you might want to question why you're building a GPL plugin anyway. But that's just me.

I just get annoyed when I see some members of the community (not talking about Carl or Joost in this case) become GPL zealots until it makes things commercially uncomfortable. All licenses have positive and negative consequences. If a person can't accept that, maybe they shouldn't be trying to sell something that is GPL.