It's probably a concession to skeuomorphism. Most physical calculators I've seen that look like the standard mode ignore PEMDAS as well, whereas pretty much all scientific calculators will respect PEMDAS.

If you took one of the people who still use old fashioned "standard" calculators and put them on a standard mode that followed PEMDAS, they'd probably be confused, at least initially. I've seen that a lot at cash registers in restaurants, dry cleaners, and other small businesses.