I'm sure the publishers/developers are giving consent because Epic most pay them a nice amount and promise them more profit (because Epic takes less cut than Steam) by making their store more popular this way.
Some games are indeed expensive, 60€ for a game, specially when many are play once and discard is insane.
Personally i'm starting to wait and see what free game shows up in the Epic store, right now I have 3 games on my library that were on my wishlist on steam! That is 3 games that I will not buy now.
About Unity, at that time it add two versions, a professional version and a personal version, the personal version was free, for non commercial games, if i'm recalling well and didn't supported many of the features of the professional version, like real time shadows, to survive Unity add to blend them into one and make the engine "free" (still not open source tho).
I'm all about choice don't take me wrong, is fantastic that many people, is now able to play games that they would never be able to, unless they pirated them, at least pirates now have no excuse. But one thing is certain in the engine space (that matter) many choices is not a thing that you have now.