To keep check on who goes in and out so unsavory things don't occur...
They lock the back restrooms at Walmart to help prevent people from stealing things, they don't have as much help to watch then and the stores are less crowded so they only need one set of restrooms.
The best way to keep them clean is to keep them locked
they are out of sight of store CCTV ofc... :P
Shítting time is during the day.