The problem with just adding free games to a collection, is that if a game is free for a limited time, you can't grab it for your library to play later, or even get updates.
You can just download it to your computer at that moment, or your screwed.
I mean, if that's itch's policy, fine. But at least add a disclaimer to free games like "You must donate at least X dollars to add this game to your library."
Until then, I'll consider it a missing feature / bug.