This is marketing for their game engine. They don't have to do it, sure. But SaaS companies don't have to provide free blogs, webinars and e-books either but we don't pretend they are benevolent for doing so.
Don't get me wrong. I think more art assets available in the game dev community is a good thing. I think more free stuff from one company (Epic in this case) will beget more free stuff from their competitors (Unity mainly). I'm just pointing out that this isn't free for the sake of being benevolent. It's free for the sake of capturing market share.