Those things don't match up like you think they do. The equivalent of /r/The_Donald banning users is equivalent to /r/politics banning anyone who posted critically of Clinton between July and the election (even though The_Donald only claims to support Trump whereas Politics is intended to be more unbiased).

These are subreddits with their own admins managing them. Twitter as a company is banning users from their entire platform. The equivalent would be Reddit as a company banning very liberal users.