Edit: I just realized you were probably talking about it being explicitly illegal to discriminate based on political affiliation in some states which is true.
https://www.cnet.com/news/six-states-outlaw-employer-snoopin...
Also I read through the text of those laws and they all specifically say that they are not limiting the employers right to monitor their own systems in any way. So Facebook looking up how their employees use Facebook would be completely fine under those laws.