Why did they limit it to settlements and player homes? I have no fucking idea. You can, technically, do the same thing in NV or FO3 using the console, it's just too tedious to really be worth the effort. Of course, there's literally no reason for this, if you get a mod you can build anywhere the build UI literally just helps you place a world object which is added to the world via script.
You can't build in those diners or metro stations, because you have to build on a predetermined (and fucking tiny) build area. Building isn't a thing I look for, but it's a nice touch when it's used in small ways to solve problems. Hell, FO4 lends itself really well to a building system - there are a lot of little nooks and crannies in Boston, like blasted-through diners or metro stations.
I would've loved to set up a campfire or a chair and some RP decor in some of New Vegas' player-home-ish locations.
