Q. Can club card records be seized by law enforcement agencies?
Absolutely. In fact, law enforcement has already been digging around in people's food purchase files -- which is part of why these records scare me. I personally don't feel like it is a supermarket's place to get involved in catching criminals, and even if I did, I couldn't support the collection of this sort of detailed, intimate information on tens of millions of Americans on the off chance one or two of them might have committed a crime.
Constitutional protections against unlawful search and seizure have (somewhat and so far) succeeded in keeping the government from digging around in the affairs of innocent citizens. But when private companies (like supermarkets) do the digging for them, law enforcement doesn't have to worry about that pesky Constitution. Let the private sector do the privacy violation and all you need is a search warrant to access what you wouldn't have been authorized to collect yourself.
Bear in mind, too, that someday the "crime catching" tables may be turned on you. Say down the road you get involved in a lawsuit and the opposition subpoenas your shopping record. Or an ex-spouse uses your file to show that you're not a fit parent. (After all, what fit parent buys condoms? Or beer? Or cholesterol-laden mocha fudge ripple ice cream?) Once information about your shopping habits is stored somewhere it will hang like ripe fruit; anyone who can get a warrant or a subpoena will have a wealth of information that can be distorted to make you look bad.
The only way to prevent these abuses of your shopping information is to make sure it is never collected in the first place.