I've never seen 'rude'. What I've seen is that when a bar gets busy, the bartenders concentrate on those who wish to purchase alcohol and will provide a current tip to the bartender's daily coffer.
...which, if we were to look at it from their point of view, we might likely do as well. If we can serve only 20 people in a given amount of time, are you going to serve 20 people who will be providing you with a cash tip on the purchase, or 15 cash tips and 5 whose tip has already been paid?
But looking at it from our point of view, we paid for a service and we have a legitimate expectation of getting it.
Quite the dilemma. We've complained about this sort of thing before: Fountain Fun Pass holders get overlooked at busy times.
Desperately seeking 10th cruise for the free laundry.