They phrase it that way to sound alarming, but the fact is that it is the default for an employer to be able to fire a person for any kind of reason, because the government has little, if any, right to enforce involuntary private associations.
If you want to be alarmist about it, you could say:
“Did you know, you can be fired for having red hair?”
“Did you know, you can be fired for liking Justin Bieber?”
“Did you know, you can be fired for hanging the toilet paper in an overhand position?”
All of those things just as true, but they don’t help push someone’s political agenda to point them out.
Well,union members don't;)