Why? There is nothing illegal about it. That's why it is usually stated in the contract.
I don't think clubs 'accept' loanees from us, certainly not talents like Christensen. They ask to take them on loan and we pick the club we feel most suites them.
You can say that it would make them less likely to want to sell us one of their own players in the future if we want them, which would be a fair point.