I think the cashier was under the assumption that you wanted the discount, and asked at the end instead of immediately after you said you weren’t already a member. It doesn’t feel rude or invasive, from my perspective, maybe just phrased as an underpaid employee.
What I do recommend is a VOIP phone number and attached separate email for all marketing purposes. They can track and sell that as much as they’d like without impacting your personal details or necessary accounts.
That spot in Kansas must be an exciting mix.