I had a Christmas job working in a retail jewelery story a few years back. I remember hating the days when major sales were on, because that meant hordes of people coming in and being rushed for my entire shift. Not that I'm excusing the rudeness of the store clerk, but there may have been other reasons for her testiness.
We also had a store bathroom, but were told it was strictly for employee use only. Frankly, I didn't like using it myself - it was small and cramped, and the light only worked half the time. Not something I'd want customers to have to deal with either! I'd much rather walk through half of the shopping centre to go to one of their public bathrooms.
In another related note, regarding holding the change rooms. I don't think it's ever really acceptable to 'hold' a change room for yourself if there are others in the store, and especially if there is a line. It screams entitlement to me and grates me entirely the wrong way.
Btw Mondrian, I'm curious - what type of store did you used to work in when you worked next door to this clothing store? I assume your store let customers use the facilities (since clothing store just kept sending them over!).