It doesn't matter, they don't have to explain their reasons any more than you would have to explain your reasons for throwing two people out of your house because you didn't happen to like the color of their hair at that particular moment.
Completely irrelevant, they are under no obligation to apply any restrictions on what they find acceptable on their property in a consistent manner, they OWN the property and can toss anyone they choose off of it just for fun if it pleases them.
What happens if the guy is enforcing the rules selectively against a certain group while allowing other groups to violate the same rules?
Nothing since it's well within the property owners rights to do so .... as in if you wish to throw people off your property for discussing Christianity and not throw people off your property for discussing Islam, then you have every right to do so.
What if that group he was enforcing the rules against were black and they let every white person get away with the same stuff? Wouldn't you assume that there is some kind racial prejudice behind his actions?
I might assume that, however the property owner would be in violation of federal civil rights statutes if it could be demonstrated sufficiently in a court of law that he/she was discriminating based solely on race (the propriety of which are a completely different matter and outside the bounds of the case in point). However that's not applicable with respect to this case.
This is what I am saying. He has the right to throw anyone out he wants but the motives behind it might have been less than pure.
Explain how his motives might have been "less than pure", since his motive was to remove people from his property that he didn't wish to be on his property because they were behaving in a way that he found unacceptable on his property, seems like a pretty "pure" motive to me.