If, upon being hired, the boss instructed the employees to "just take one of the mugs right off the rack - it otherwise takes too long to actually chill it," then the bartender is just following orders.
If the boss told them, "always put a couple dozen beer mugs in the fridge, so they get nice and cold - our customers love that," then the bartender would be violating orders, and should be admonished.
Regards,
Of course, if the boss instructed that customers have to pay extra for a frosted mug or to rub the lime slice around the rim for a drink or to add an olive to a martini, or to smile and be friendly, then that would be a stupid but official boss practice if he wanted to drive away customers who paid for his products and made his business possible but that didn’t slip extra cash to his employees.
I wouldn’t want my waitresses and help to pick and choose among my customers and drive off the ones who were just buying what I make and sell.