To be fair, Congress could, on a whim, change the definitions of hate speech whenever they felt like it. If you're arguing about selective enforcement with TOS, the police already selectively enforce laws. Hate speech would just be another one they could use against people they don't like, while ignoring those they do. There's no good solution.
That said, things like speed limits for example. We all on principle agree to this type of enforcement. One via the licensing and second via legislation. We could make changes if we thought it important enough.
Look at prop 47. People erroneously thought we were prosecuting too many minor thefts. So the law was changed. Now many people are looking to reverse it given the unintended consequences. Point is, at least we can affect legislation directly. Not so with ToSes unless Congress takes up the issue -which so far they are loath to do.
In the US, the legislature cannot unilaterally create rules that outlaw certain speech (unless that speech falls outside of the guarantees of the US Constitution). Also, there's no current definition of 'hate speech' in federal law.
With a ToS you can make it up as you like. Speaking against having pets could be hate speech against pet owners. I mean it could mean whatever the mods want it to mean. You can circumscribe it to only affect people you want affected for whatever reason.