He owns the code.
He therefore has the right to sue if you're not compliant with the license.
I can't say I disagree with his stance - maybe his intentions, but he's not doing anything wrong as such. He's probably annoyed the people AREN'T honouring the GPL on his code. And 0.25% is a DAMN SIGHT more code than I'll ever wrote, own, or get to be used in a major modern operating system. He's done the legwork, he's not exactly trolling, and courts are presumably not throwing out his cases. If he gets a few million from companies because they're not complying, well done to him. I'd like to think if 'twere me that I'd give some of that back to Linux somehow as it's not just his code that was infringed, but that's a personal, moral choice, not a legal one.
And these companies presumably infringed his code that GAVE NO SUCH RIGHTS to allow them to resolve their GPL infractions in a nice way. Even with such a statement, there's nothing to say that every kernel developer / copyright holder agrees and will abide by that - they can't. Some of them aren't even around any more to give that kind of permission.
Sure, it's not what I'd do. But several million Euros for plainly illegal infringement of your personal copyrights? Yeah, I'd be having words for sure and that would give me one heck of an incentive.
I can't say that I could really make the guy out to be inherently evil, as the article seems to imply I should.