Basically these reasons: we have been victims of censorship in exactly the same way. Even Vichy which don't want any societal change have. Others in other causes have too: that's why I mention Trump and Minaj, and I have many more examples. Again Burma did it with Zuckerberg and zomg that is SO WRONG! Finally, again, it is hypocritical to the extreme that they are censoring Russia but they allow China with its slavery camps, Saudi Arabia with its Yemeni genocide and its primitive legal and political system or groups such as again cartels and ISIS.

You don't have to be Putinist or side with Russia now to see that this sets a very dangerous precedent. In the case of US sites, it also puts into question their common carrier (rather than publisher) status.