I think you are seeing a white wire connected to your existing switch and incorrectly making the assumption that it is a neutral. The black wire coming into the switch is likely the hot wire from the breaker. The white wire from the switch is going to the light. The white wire should have been marked with a black mark or piece of electrical tape when it was originally installed to mark it is being hot. This is normal when installing NM (romex) sheathed cable.