Why? They're complying with the license. It shouldn't be surprising to see them taking advantage of copyright law when it benefits them, then turning around and taking advantage of copyright law them it benefits them. It's quite a bit more benign than Disney making movies off of public-domain stories while lobbying to keep Mickey Mouse under copyright, since they didn't even lobby for it. The WordPress developers just gave them the code for free.

Free and open-source software isn't subversive. It's innovative, which isn't the same thing. It's certainly a new model, but the way you get a GPL violation removed from GitHub is with a DMCA takedown notice. Free and open-source software is a different and nicer use of the copyright system, but it's all still the same system at the end of the day.

And now that they've had a couple decades to get over their discomfort and they've realized that there's nothing subversive there, people who love copyright are totally fine with free and open-source software, because it furthers their goal of making money.