HEVC costs money to license iirc

So you're telling me that M$ can't afford to give users that paid $99+ for the privilege to use Windows 10, a free license for native HEVC support? 

yes

windows 10 itself is free(ie, the iso), but the license is not (to activate it, get rid of the watermark) or something like that

the videos app is bundled within windows 10 itself, preinstalled, I think they would have to pay royalties or something everytime, not sure though

IDK, hevc files work fine with the default microsoft video player, and i haven't installed anything.

Get that media player with the traffic cone, it’s the best and is free.