Has this guy on a random message board said one thing that is untrue? Even so much as one thing? Show it to me. In fact you are talking about "a guy on a random message board" because you know that what I am saying is true. Everyone else reading this thread knows it is true as well. If you could defend your behavior, you would. You can't so you attack.
This is not about "manners'. Do you think it's pleasant to get yelled at by some random guy in a store who is yelling at you simply because he knows you have to take it? Who is irritated because of the offer you are required to make and wants to make someone suffer for the sake of his irritation? Think. It's not all about you even if you are a customer.
Allow me to mediate. The thread is supposed to be about hardo things we do, weren't we in the trust tree wherein we aren't attacked for our hardo behaviors? Although the title is the hardo things you do out of 'principal' [sic] (yes I know UCDaveD corrected, nonetheless out of principle I must point out the error).
The principle here is, don't offer me unsolicited memberships every time I buy something.
Weighed against the principle of treating workers doing their job with a modicum of civility.
The way to effect change may be to tell the cashier; "I don't like rewards programs, please don't ask me and tell your manager I don't appreciate the solicitation." Being sarcastic to the cashier may change an individual cashier's ask, but it is less likely to change the practice in general. I think to issue a final ruling we need to know if Deepster is a member of any rewards programs.
If the hardo behavior is out of a principled stance to not give out information & avoid all reward programs he's got a point. Maybe he can modify his reactions to increase the odds the questions aren't asked, but a hardo's principle to avoid rewards programs may supersede the principle of civility. It isn't great, but not as dangerous as the other hardos swerving in & out of traffic to enforce their principles.