One would imagine such a practice to be illegal. But yeah this was my question as well.

I presume an internal (not shared with others) "do not hire" list. An internal one is perfectly legal.

Most companies aren't going to help their competitor. Bad actions from candidates have consequences.

It's telling when you jump to 'ilegal behavor' yet think acting like a total unprofessional asshole by wasting everyone's time with a company doing the right thing, is totally acceptable.

I would never ghost a company or a candidate and expect other people to have the same courtesy.

It also shows that bias is perfectly acceptable against groups you dislike (companies you think are all acting the same, so they all somehow deserve the same shitty behavior) yet unacceptable to a group you support.