Because it encouraged people "to bring your whole self to work", and during previous years it supported and encouraged employee being vocal and protecting about certain issues. They like to "amplify" certain voices and suppress others, so to speak. It's in their DNA both as a company culture as well as their main business - moderating social sites.

What some employees haven't quite learned yet is how to read between the lines. They are certain topics which are ok to protest and be vocal about, but others, are not, like unions or this case. To be safe, it's better to think of "bring your whole self to work" as a gullibility trap. It may not be that, but effectively it will function as such. It catches the naive troublemakers so later on they can be filtered out.