Because people think it’s funny and imgflip is pretty much just less popular, politically right wing reddit. There has been an issue a lot of stuff they haven’t addressed. This rule is pretty much unenforceable since they can just take the streams off public and no one can see them, but the streams will still be active for those that follow them. The only way to report the stream in that scenario was you got invited to the stream via a link or followed it before it went private.
The issue for this is those streams tend to be very popular and I think the global mods anre afraid of losing users even with the large influx of toxic kids during covid-19 pandemic. Same thing when these streams advocate for the removal of other streams unless the stream is full of outright NSFW images or generally extremely racist.
The real reason? I don’t actually know, I can only speculate. Probably because Anti-furry streams I know are as popular as the main furries stream, so why get rid of something that keeps users here? They get more ad revenue from it.