All I could afford wrote:
Most likely equipped with a vacuum breaker in line to prevent back flow. There is probably a small sticker with tiny print worn away stating that. Some dripping is normal upon turning off the hand held hose. Continuous leakage usually means replace the valve.
This. They're designed to leak for a while after use. Only if it's still leaking a day later do you have an issue.
The vacuum breaker, which causes this, it a safety device. It prevents water from siphoning back into the fresh water system if the shower head were to be dropped (and left) into gray water. Not a very likely occurrence to have a tub/basin left full and the shower head left in it, but possible. Welcome to nanny government, which requires manufacturer's to protect you from yourself.
