Memories are a funny thing and there is something to be said for IDing regular customers. There is so very much at stake for these bartenders and the business that there is little excuse for not IDing someone no matter how many times you think you have seen them.
Confession time: When I was 20 years old, I accompanied slightly older friends to their local hang out. There was a bouncer at the door checking IDs. My friend was a regular and convinced the bouncer that I was 21 and had just forgotten my ID. Once in, nobody else checked. The following week, someone else was at the door who asked for my ID. I saw that the employee who had worked the door the previous week was now working the floor. I pointed to him and said, "he saw my ID last week; he'll remember me." That employee "remembered" that I was 21 and vouched for me. The following week I had two people vouching for me - all convinced that I was 21 and that somewhere along the line I had proved it. I celebrated my 21st birthday there months later. The band pulled me up on the stage and asked how old I was. You should have seen the looks on the faces of all the servers, bouncers and bartenders when I proudly announced I was 21 that night.
Even if they are sure they know you and are sure they have seen your ID in the past, memories are tricky and cannot be trusted. They should always ID. A shopper's rotation should have no bearing and is a good test.
There are two types of people in this world: Those who can extrapolate from incomplete data.