I know, that‘s because the rule isn‘t on the stream but on
imgflip.com/terms
As said in the conduct paragraph: „In any content that you create or modify while using Imgflip, do not discriminate against any sect, commit or condone illegal acts, harass others, or otherwise make it less fun for other people.“
Now I‘m not very strict about the „make it less fun for other people“ part, but death threats are where the fun ends.