Well, I've been working as a hospital cleaner, working on some wings or departments (?) that were full of elderly patients some of who started to go senile at old age, and at least a couple of them had difficulty accepting that you're supposed to go crap in the sparkly white clean-smelling disinfected porcelain-filled room, and instinctively went to take sneaky shits in the garbage collection room. (guess who had to clean it up? yeah)
They'd lived their childhoods in the countryside without running water and were used to an old-school outhouse that smelled like shit, see, and so they felt that the place that smells like garbage (and old diapers from the bedridden) must be the place to do your business.
So if Star and River are from a medieval-ish society without running water, they'd probably assume that the place to do your business is in a separate craphouse that smells like it, and not at the fragrant, shiny bathroom where you cleanse yourself.
I mean there's been shithole toilets since ancient Rome. Mewni must have some toilet systems somewhere, it's just that they don't waste clean water to flush.
Star probably asked Angie sometime during the first day about it. Or maybe she's just been magicking her shit away all this time because she wasn't sure where you're supposed to go and didn't wanna ask