Picture a shower head as the end of a garden hose just because the nozzle is supposed to be off doesn't mean it wont leak.

The shower head lets water out and possibly allows for various spray patterns PERIOD.

It is not intended to act as a shut off as this would then be a mixing device allowing cold water to enter the H/W not a good thing as this is called a cross connection.

What does close the water is the shower body and sometimes with age the cartridges / spindles slip allowing seepage and these are the parts that have to be replaced.

I had several American Standard faucets in one account turn themselves on.

This is especially true when folks are not home and the pressure builds inside the system like a hot water heater where no provisions were made for the expansion during the heating cycle.

Get the name of your shower body model number if possible and order the repair parts as instructed.

Good luck.

Sylvan Tieger,LMP