I'd suppose it depends, if you are under time constraints you might have a hard time, or if it's past typical business hours or, probably most likely, you just really don't know the area. They will send you to any small town or city within 75 miles or so of the station.
But, no, generally most business' will not allow you to use their restrooms. Some do, most don't or at least most would at the very least have the rooms locked and you'd have to ask for a code or get a key.
I'd imagine things would vary across areas, being as it's seattle and the surrounding areas, pretty much everything is locked down or surveilled.
Also, as the above vid shows, drivers aren't always very bright, that girl is dumb as shit to do it without even thinking she'd be on someone's cam. Sadly, a lot of drivers do a lot of dumb shit and people think we're all just as dumb.