I mean, beyond the personal political opinion of Mark and shareholders, why would this company defy those governments?
(asking naively because I feel blinded by my opinion, I can't make a good strategic case for it, only a moral one)
Advertising, market segment/product substitution.
There's a pervasive idea that Facebook must be gobbling up your communication data and it's starting to weird the general public out. It's very common to see people discussing how their phones must be recording for Facebook because they said something near their phone and then they saw a Facebook ad for it. Making a big publish show of fighting the government over whether messages can be intercepted at all provides the kind of advertising that money can't buy.
Also, there will likely always be some subset of people who are more technically savvy and care more about their privacy. These people tend to also set technical trends for the rest of the population. People may leave your platform for being too insecure, but nobody is going to leave your platform for being too secure, so build a secure product and target both market segments. This will give you a larger market, and will also deny your competitors the opportunity to grow by luring away a subset of your users by offering what is to those users a superior product.
So when companies like Apple are offering great security, people are scared Facebook is reading their conversations, continuously tracking their location, and listening to their microphones.
It would also let them avoid inevitable articles such as "Facebook helps Beijing/Erdogan/Putin spy on protesters". I'm sure an executive could argue those drive away some users.