As public as he is, Warner's would obviously know of them, so there's no "Learning about them"
Yes, someone who is in the KKK has a right to be employed. They do not have the right to use a job to commit prejudiced acts though. And, if they were hired, and the position gives the possibility of performing their job with bias against their particular group they hate, they should be watched and warned it won't be allowed. It's not up to me to say who anyone hires, if that person abuses their position, then absolutely it becomes a problem that shold be protested and/or fought against. Now, of course, depending upon the degree of the potential problem, many wouldn't hire them, but, it's none of my business until thy actually do what I suspect they'll do.
McCarthyism wsn't just about Govt rounding up Commies, it went much further:
Fred was arrested last week for being a Commie
Joe, his next door neighbor was good friends with him
You had dinner at Joe's house last week, so, you must be a Commie, sorry, I can't let you have the job and I'm sending this information to the FBI