On paper, universities disallow helping others cheat. In reality, it doesn't seem to be enforced.

If the helper student has already passed the class, the professor can't take disciplinary action against them without making a major case about it. It's easy to simply fail the cheating student or give them a zero on the assignment without making a record.

Additionally, if it is brought to a committee, it'd be much harder to prove.

So although it's officially not allowed, there's not much universities are doing to combat the providers.