Nothing, just like nobody stops people from releasing modified builds now in fact people do (waterfox, ...). Trademark law provides a moat, as do fees to get into proprietary app stores.

Nothing, but it doesn’t work for closed source software either.

Pirating a FOSS app would probably have less of a psychological barrier, but I think it would be more frowned upon instead. (I won’t judge you if you pirate Photoshop, but pirating Inkscape? Shame on you!) We’ll have to see how it plays out though.