I can't say what the precise circumstances were in your case with your operators (e.g. what was the tone and attitude of the bank staff, etc.), but I think there is perhaps a valid reason why banks ask such questions in general.
It is not to discriminate the low income group, but rather to see if there is anything suspicious, because some dark organisations are taking advantage of these simple folk by using their names to open savings accounts and then using the accounts for money laundering purposes.
It may or may not be the case during the encounter you mentioned above, but I'm just saying that logically, a regular person (i.e. a normal bank employee) wouldn't really care whether the person who walks in wanting to open a new account, is a low or high income earner, etc.