Hopefully I can shed a little light on this, being from the US and now live in Denmark.
The culture in the US (which dominates reddit) is one of promoting this vague concept of "freedom of speech", even when using that concept is mainly to hide your true motivations.
We're taught from the time we're four and five years old what a great country the US is because of freedom of speech. Therefore, it's in our minds whenever someone tries to "censor" us that we should fight back, no matter what the situation.
Now, there's another rising movement in the US that fat people should be discriminated against. You'll hear the claim that "it's not like racism" because fat people could potentially get thin again.
Another cultural aspect at play here is the large class-based system in the US. Because we're much more right-wing/conservative than Europe, we are taught that to succeed in life, you need to rise to power. In cases where that's not possible, we tend to make ourselves feel better by putting down other people as a way to be "better" than them.
So now combine those three concepts. Reddit banned FPH, which caused an immediate reaction of "my rights are being trampled!". FPH was the wrong subreddit to start with, because of the movement to discriminate against fat people, as well as the need to have someone to discriminate against without making yourself feel bad about it.
I don't think anyone anticipated this level of reaction, but once a few people start shouting about how others are being put down, the groupthink (people not thinking for themselves) becomes overwhelming to the point where we are right now.
I'll leave the implications/legal stuff/future of reddit for other people to discuss, but I thought I could at least contribute with that.
(sorry for the English - I'm very much a beginner at Danish)