Nominally it's a bandwidth issue, trying to save those poor beleaguered telecoms that have been forced to offer unlimited data to their greedy grubby jerkwad customers some money so they don't have to tap into the billions of dollars the government gives them just for existing. (Lot of sarcasm there, just in case, but the point stands)

I don't know why they can't cross check to see if you're on wifi or mobile data before restricting you, though.