This is a pedicab service. A cynic would assume that Pedal Me is mandating this (and thus putting their drivers at risk) because passengers are more nervous about taking a service where the driver wears a helmet and they do not.
This might lower risk for the customers because driver may take less risk without a helmet in some situation but the risk for driver is only increased.
This is optimizing for customers at the cost of drivers.
I don't see any supportive extensive research linked from them .