That's not necessarily true. I think most people, including me, assume that most retailers have some kind of coupon system and don't need to see a coupon field to think about coupons.

Anecdotally, there's been cases in the past where I see a product as valuable but overpriced, and a coupon makes the difference between a sale and a pass. I will go to the checkout screen with the product to try some coupons and see if they work, not having yet made the decision to proceed with the purchase.