They switched the neutral, so hot runs to the fixture, then to the switch, when the switch is in the off position the circuit can not complete path to ground, in the on position it can.
OP should also be able to confirm this by removing the bulb. There should then be no voltage detected on the switch wire, regardless of on or off position.
Exactly. OP can unscrew the fixture and will probably see hot black to fixture, white to white, and black coming back from the switch to the fixture. It's an easy fix. Hot black to switch-leg white, returning switch-leg black to brass screw on fixture, chrome screw on fixture to neutral.
Your switch was wired "backwards". Easy mistake to make and to fix. Singe pole single throw switches work both ways, so there really is no backwards. If it were a different kind of switch you might be in trouble. This not a danger to you unless you have wild OCD.
spedfarm t1_j6jhb8x wrote
They switched the neutral, so hot runs to the fixture, then to the switch, when the switch is in the off position the circuit can not complete path to ground, in the on position it can.