Communists aren't associated with violent activities in the US in the recent past. As in violence within the US. Nazis/Neo-nazis are. So saying communist would be a political statement as it doesn't pertain to safety while Thug/Nazi is about safety.

I personally consider both "keep safe, there's communists about" and "keep safe, there's trump supporters about" to be political statements that may be against company policies.

I had to use Thug - because Thug is derogatory for everybody.

Communism is only derogatory for the few people that understand history