If they paid to get listed there in the first place (paid link for the purpose of better rankings), then it's fair game to pay to remove the ranking disadvantage now. I call it an ethics tax.
Paid links were frowned on but not generally penalised. So sites took the risk they wouldn't get caught or penalised. Now they got penalised. So if it was okay to pay for a linkback, equally it should be okay to pay for the removal of a linkback.
Perhaps. I don't have much of an opinion one way or the other on if it's moral to charge for something like that. But the Disavow Link Tool exists for a reason, and webmasters can use it to manually remove those bad links. Though they are encouraged to make an honest effort to contact webmasters first.
Paid links were frowned on but not generally penalised. So sites took the risk they wouldn't get caught or penalised. Now they got penalised. So if it was okay to pay for a linkback, equally it should be okay to pay for the removal of a linkback.