I have more or less stopped following the amount of time they provide, as long as it is close, but if they cut 15 minutes out of an hour, that is obvious. So business is business and a deduction in their fee is fair Business. Figure out the dollar per minute fee, calculate the cost of what you were shorted, deduct it from your next visit, or from that visits tip.
As Dylan wrote, Money doesn’t talk, it swears! And you will probably hear them do just that.