I'm also always polite to call centre workers (a simple No thankyou will usually do)

I'm not. They call me in the middle of my working day when I am heads down. I am not sitting at home eating bon bons and watching soap operas. I am working. Most of them do not even have the courtesy to ask "Is now a good time?". I usually just hang up on them, unless I am particularly frustrated, trying to solve a gnarly logic problem. Then I yell at them before I hang up. I know it always make me feel better <vbg>.

and to people on the street collecting and selling. I know a lot of those jobs are not great and my children have done some of them for a time.

Last year Andy and I were in Alicante and he forgot to bring his sun glasses, so we stopped to look at some of the sunglasses that a street vendor was selling. I really thought that he was going to assault us when we tried to walk away without buying anything. He actually chased us down the street! I do not feel the need to be polite to people like that.