Apparently it's not just an American thing, but maybe other countries have more sense not to do it anymore.

They're usually in "high end" restaurants in big cities like Las Vegas. The ones I recall usually have the sinks somewhat separated from the stalls with a partition or turn, but they're not wholly separate rooms. The motivations are probably more needing money, access to a fancy place, and being an extrovert than perversion - more windshield wiper gig than peeping Tom.

I think it's a combination of a holdover from another time that maybe was useful when they had an expanded role - they probably actually used to keep the bathroom clean, and some guys will shine shoes etc. - and tip-based service jobs they gave to poor people. I think they do get an hourly rate, but it's probably below minimum wage for the same reasons waiting tables is.