Everywhere that I've been the purpose of Park&Ride lots is to reduce the number of cars on the road - use of mass transit, carpooling, bike/car, bike/bus/car, etc. have all been legitimate uses of the lots.

OTOH, I'd be more concerned about using private parking lots at stores. Many of them are ok, but in areas where parking space is limited they sometimes enforce parking restrictions for customers only and may have cars towed.