Well, they use PIN codes. The locker locks, and even you can’t get your phone back until you enter the code. You don’t worry about security when you leave your belongings in an automatic storage locker at a train station, do you? It’s the same thing.

The only thing is that you have to pay for the locker. At least in my city, at the train station, you get a receipt with a barcode that you scan later. If we're talking specifically about phone charging solutions, they usually use a PIN code, and the security comes from the fact that the PIN is created by the person who owns the phone. The system doesn’t generate it for you. If you're really concerned about security, try asking the companies that provide these charging lockers and solutions for more details.