Hello "You have to have original fnaf 1 on Steam to be able to play the game." No pleas No
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If that wasn't a thing, then they'd end up with even more legal issues than before.
This is actually an established thing w/ FNaF fangames, as a while back there was a fan remake of FNaF 3 that Scott Cawthon asked to be taken down due to it basically being a free version of the original game, to the point where it could technically be considered piracy.
TL;DR, further fan remakes like this are always gonna have to have checks in place to ensure that you own the original game.