Are you implying those people are auditioning ironically and know they are bad singers? - No. I have absolutely no idea why you ask that.

It does not seem rude to me to try and change the course of someone's life who is _drastically_ underperforming. - Leaving aside what underperforming could mean - so, if he means well, it's just not possible for Cowell to be rude, no matter what he says? I think that's just not what the word means to people (apart from you).

That doesn't follow the implication that rudeness shouldn't happen; - I can't parse that, sorry.