No. Nothing reddit is doing is illegal. When you sign up for a service like reddit, there is a TOS, which allows them to ban, warn, limit interaction, etc, at their discretion when terms of their TOS are violated.
If their TOS doesn't allow pictures of butterflies, and you post pictures of butterflies, you will receive a warning. Continue posting butterflies, you'll get banned, until eventually receiving a permaban. There is absolutely nothing illegal about it, because their TOS specifically states no butterfly pictures.
If you're stupid, yes. If you have half a brain, it's not.
If you play in the MLB and take steroids, you get suspended, and if you keep taking steroids, you get a lifetime suspension. Steroids aren't illegal. That's not censorship. It's breaking the rules and facing the consequences of breaking the rules.
If you're stupid and think facing consequences for breaking a rule you agreed to is censorship, that's on you for being stupid.