I don't say Blizzard has broken any law. It hasn't. I just tried to remind people that Blizzard is a private/corporate entity that will not endorse political debate because it may hurt its commercial value.
I'll add that Blizzard is absolutely right in doing so. Because, it clearly can't be an open space for expressing opinions. That's because its values may be in competition with those opinions. So any opinions expressed inside Blizzard stuff would be "tainted" by Blizzard's nature/control. So Blizzard wouldn't be very good at letting opinion being expressed. So Blizzard's decision actually helps to maintain the expression of opinions in place where it is most efficient : the public places, the parliament, etc.
And yes, that holds for FaceBook, Google, etc. Much of Internet is privately held => expressing public opinions over the web is tricky.
Now, following you reaction, I've checked the Universal Declaration of Human Rights, specifically article 19 :
"Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers."
Which basically says that Blizzard should not oppose the expression of opinion (provided one interprets "media" in a wide way : speech is a medium).
I admit that the more I look into it, the harder it gets...