It's a quality issue. Beer served in a new, spotlessly clean glass will have a distinct foamy head on top of clear liquid. If the glass is dirty or scuffed, bubbles will rapidly form on the sides of the glass, which seriously affects the quality of the beer. The head will be thin and uneven, the liquid will look murky and the beer will taste flat.
This isn't meant to be a nationalistic point, but America has a relatively undeveloped beer culture; beer is often kept and served very poorly and drinkers rarely notice. British, Belgian or German drinkers will notice, complain and tell all their friends if you aren't serving beer properly.
https://cdn.shopify.com/s/files/1/2971/0116/files/dirty_beer...
This isn't meant to be a nationalistic point, but America has a relatively undeveloped beer culture; beer is often kept and served very poorly and drinkers rarely notice. British, Belgian or German drinkers will notice, complain and tell all their friends if you aren't serving beer properly.