They’re trying to capture the consumer surplus. Normally, a seller can have either high margins and low volume, or low margins and high volume. The retailers wet dream is to get the benefits of both. If the reward program profiles you as someone who buys coffee at $4.00, but not at $6.00, you’ll get coupons for coffee that the people who buy coffee every week regardless of price won’t get.
FWIW, I’ve found stores that don’t even have rewards cards frequently have lower prices than their competitors’ reward card sale prices.
Party affiliation as listed on voter registration isn’t a loyalty oath either. It’s just a way of asking which primaries you want to vote in. You might as well pick a party with relevant primaries in your area whether you agree with their platform or not.
(Voter registration is public information and may affect which candidates and PACs send you mail.)