One point: Shower head (and Toilet, and dishwasher, and pretty much everything else water related) is restricted by EPA regulations, not by law. So he's not trying to change law, he's trying to change the EPA regulations.
Since the EPA is an executive branch department, which works for above mentioned executive branch head, he has every right to tell them to change the rules because he doesn't like them.
OKay, that was more than one point.