They're not attacking free expression but trying to reign in abuse of their platform.
Nobody really knows what "liberal" means because it's been used in several very opposite ways in different times and places.
Just google – the_donald upvote script – to get an idea. Several tools were released for users to upvote or downvote everything targeting a subreddit or user. It's a sort of grassroots vote manipulation scheme.
This whole witch hunt/conspiracy started and kept going with trolls from the_donald. People getting death threats from getting doxxed on reddit threads should not happen. Also, during this trolls kept accusing spez of being a pedophile. While changing user comments is extremely inappropriate, I think that the_donald has some extremely toxic trolls. So I'm definitely ok with reddit admins shutting some of that activity down. I would have found it completely understandable if reddit had shut the_donald down with that with the whole pizzagate thing. But that sub has a ton of activity with a lot of politically active posters, and so I don't think reddit really wants to shut it down.