If I remember correctly, IG attempted to make non-NSFW content NSFW. What is and isn't NSFW is determined by imgflip itself, with the parameters of NSFW content being one of the automatic rules listed on every stream. It is not within a stream's jurisdiction to change the boundary of what is and isn't NSFW, as it would contradict the rule imgflip has already established; the rule that is in fact listed in every single stream's list of rules.
However, it is well within a stream's rights to decide what sort of content is and isn't allowed on a stream. This includes the restriction or banning of content that may be within TOS. For instance, politicsTOO generally bans right-wing content. Even though such content is within TOS, the stream has chosen not to allow it, that being a stream's right. And again, look at any stream that unfeatures unrelated content, even though said content is well-within TOS. I'm not trying to do something that every other stream isn't already doing.
What's unconstitutional about it? Freedom of speech doesn't protect indecent exposure.
And if that's not enough, the ship has already sailed on the infallibility of the Common Sense Constitution
imgflip.com/i/76rm9t (see Article 11 Section 2)