A person is out and about town, and suddenly has to go, bad. They pull out the app and find their closest Rent-a-Potty. It appears on their map with a green dot - that means the last person to use it considered it reasonably clean, a good sign. (Everyone knows that people whose usage of the Rent-a-Potty is, ahem, strongly correlated with the state transition to an orange or red dot face price hikes, or possibly even expulsion from the app as a whole, so there's good reason to leave it at least as good as you found it.)
They tap the dot, hit "Reserve", and the timer begins. A 4 digit PIN code appears on the screen, your secret incantation to the hall of the porcelain throne. These things are not cheap - $1 per minute for the first 10 minutes of reservation, $2 past that. But they're emptied every night, the sink always has hot water, and the soap is always refilled.
Some people have tried to game the system by {your deviancy here}; the company has responded with {countermeasures you're smart enough to think of here}. Some enterprising souls could theoretically {countercountermeasure}, but honestly, implementing {countercountercountermeasure} isn't a huge concern for Rent-a-Potty's C-suite, compared to the recent competition from new market entrants like Loolurker, AirPnP, and Pouber.