It's not a punishment, it's to prevent nepotism and moral hazard. I think normally they will try to place you in the same role in some other project but that may not have been possible in this case(?)
The way you implement that is by proactively asking for a transfer before the relationship comes to light. If you don't do that, this is essentially proof that you were seeking to abuse the situation in some way hence penalties (on the supervisor, see my side comment) are very much appropriate.
It's not down out of cruelty, it's done out of greed. He's more important, so even though he should absolutely 1000% be the one on the chopping block, she gets forced to sales. Then he uses the financial hardship and promises from him to get her to quit.
How to tidy up a disastrous bit of professional misconduct in just a few easy completely sociopathic steps.
