If you have a Bank account  you should have sufficient knowledge to know that general transactions are to be done at the Tellers counter. The person asking you is being courteous as there are many people who do not know whether to go to the service counter of the tellers counter.

Also each individual teller generally has their own log in , so it is not that easy for someone to just jump on "the counter"

It is not a profound lack of manners, unless someone is being disprespectful. The reason they are asking the people behind you is to ascertain if one is in the wrong queue r if they have a transaction or request that does not require a teller. Yes I know what I said about general transactions but all too often people with cheque deposits can drop the cheques in an express deposit box rather than standing in line.

Customer service relates to the manner in which you are treated by an organisation. It does not mean that those working for Banks or any organisation are required to listen to the sarcastic and sometimes rude comments customers make.