This hit on one of my thoughts and suspicions. The tenants probably were informed about the chargers, but even if not, have the ability to enquire and complain on behalf of their guests, assuming the OP was actually a guest of someone living there.
It might be that the intention was for it to be free for guests, but got used by the general public as word spread about the free charger, so the apartment owner reacted by charging for it out of self-defense without thinking about consequences, probably regretting the decision to put it up in the first place. Is it possible to somehow provide the guests with a way to communicate with the charger to enter some code to make it not charge?