Despite surface similarities, there are big differences between the British Pub and the American Bar. Brits, imagine this scenario:

You enter a drinking establishment and head to the bar, where you order a pint of something. The bar-person gives you your pint and informs you that the price is three groats. You give the bar-person three groats and retire to a table nearby where you consume the drink whilst reading a book. After you have consumed the drink you return to the bar and order an additional pint of the same. You return to your table after paying the three groats.

Brits may be surprised to know that in some of the bars I’ve visited in Philadelphia and its surroundings such a simple set of events could have made you the subject of hatred from the people that work there!
There are at least three faux-pas’ in the above scenario of which most Brits would be unaware. Nearly all of the faux pas’ stem from the fact that serving staff in the US rely, and I mean, rely on tips. So for starters, paying three groats for a three groat beer would be very rude. In Philly as I write this, the general rule is a buck for every drink ordered (caveats to follow). For full service, the tip should be 20% if the service was acceptable. Any less is an insult.
Secondly, if you order drinks at the bar, the bar-person gets the tip and not the person who serves your table. This can cause all kinds of problems. There have been times when our allocated server came up and berated us for ordering drinks at the bar even though we’d never seen her before.
Thirdly, we didn’t wait to be seated by the maitre’d in this example. A certain chain of North American “traditional English pubs” has the dreaded sign outside each of their premises that reads “Please wait here to be seated” (in fairness their food is authentically crap). If the maitre’d doesn’t seat you, you’ve broken the restaurant/bar. God forbid you buy a bunch of drinks at one table and then decide to move to another table when a new group of friends turn up. This can cause serious aggro over here.
Finally, we didn’t order food. We just chose to drink two pints of something. Even though the mark-up on drink is better than that on food, I have been subject to derisory comments, and witness to many more when the server considers our purchases to be too small. Every time I enter a bar for an after work pint and get asked if I want to see the menu, I get saddened.
Obviously there are places here not so strict and also not full of uptight wait-staff, but these places are rare. And in fairness if it’s all based on tips it’s understandable that they’d be uptight.
There are times when I yearn for the poor but direct service of British bar staff who only get annoyed if you can’t order a drink quickly enough. Once served, the transaction is over and you can drink wherever you like. With, optionally, a packet of pork scratchings.