Charles said:
It's Simon Cowell's show.
If he wants to do something he can.
He can, but he could also be sued for breach of contract (guest performers of course always sign contracts before performing in this way), he could be boycotted by artist groups, he could be dropped by advertisers and music labels (imagine if no contestant could sing or perform anything owned by Sony), and he could just in general be ostracized and blacklisted by the industry if he were to be harsh and critical with guests.
If someone came to your house and smelled bad, you
could say "holy s***, you stink like a pig." It is your house, after all. But he could leave and not come back and tell all of your friends what an a*****e you were to your
guest.