I think your guess is correct. GPL/LGPL is a contract that puts certain obligations on all parties; just by being a redistributor, Unity now becomes a party to

I think your guess is correct. GPL/LGPL is a contract that puts certain obligations on all parties; just by being a redistributor, Unity now becomes a party to a contract they never signed or even intended to sign. It's easier for them to refuse to be a party to that contract by not allowing any software under that license on their property to begin with.