Most remixes only use samples and small sections of the original song, while nightcore pretty much always uses the entire song.
quite literally every remix i've seen uses the entire song, makes a different 'sound' out of it, and kind of shuffles sections of the song sometimes maybe?
its theft because its taking an entire song, speeding it up, and claiming it as something else
explain to me what you mean by claiming it as something else. i'm fairly sure they just do the whole "forgetBOY FRED - SLUMJAMS [NIGHTCORE EDIT]" thing and not "NIGHTBOY NED - NUTJAMS"
if they claim it as their own, original song, NUTJAMS, then that's probably theft