Gifts definitely confer obligations on the recipient. You can experience this firsthand: take the next gift a loved one gives you, and then sell it, and let them know. Please report back on how you selling their gift impacts your relationship with that person.

People can license their code however they please, but comparing open source software to a gift is not an argument for permissive licenses.

True, but you would probably still be pissed if somebody took your gift and hit you over the head with it.

It does. There is an implied expectation that the recipient will will not be selfish. They can pay it back, pay it forward, possibly later when they can afford it, etc., but they are expected not to be selfish and also give someone something eventually.