I'm still not sure we're talking about the same thing. I have a Visa credit card, and I have entered the same card into Apple Pay. The card, like most in the U.S., does not have tap. It's swipe only. At the grocery store I shop at (being the closest to my home) I can swipe my Visa card through the magnetic reader, and it works. The check-out lady said they do accept some forms of mobile payment, but with Apple Pay on my phone, I was unable to pay. So she made a phone call and told me that their store does not yet accept Apple Pay. She has seen people pay with their phone, but she could not tell what sort of phone, or what payment service they had.
Apparently, the stores make individual contracts with each different mobile payment service. So as an example, they might accept Google Wallet but not Apple Pay.
Since I carry my card in my shirt pocket, it's actually easier for me to swipe my card than to pull out my phone, double-click the home button, put my finger on the ID reader, and hold the phone up to the machine. The trade-off is that presumably Apple Pay is safer.
Stores have no contracts with these providers except for marketing. I imagine the people who paid with their phone at that store were using Samsung Pay which can transmit using a magnetic field to the magnetic stripe reader.
Apple Pay and Android Pay use contactless card emulation and should work wherever a contactless card works (barring random compatibility issues, as I noted). Look for the EMVco contactless logo, one solid green light (or on screen depiction of a light), or the phrase "tap card" to know where you can use Apple Pay and other contactless card products. Unfortunately, this isn't a guarantee for various reasons. CVS, for example, has enabled contactless readers with the EMVco logo and a solid green light but when you tap you get a contactless not allowed message (it used to work, but they disabled it when Apple released Apple Pay to keep people from using Apple Pay). Toys R Us and H&M display the logo and the tap card wording but when tapped prompt for a card insert because their terminals are misconfigured and do not support EMV contactless. They may work with some cards in Apple Pay, I've only tested with physical contactless cards. Apple Pay sometimes, especially for AMEX, is setup to use an older contactless tech that may work. Testers wanted. Toys R Us worked perfect until they enabled EMV.
These compatibility issues have nothing to do with contracts though (except when they block all contactless intentionally like CVS), and everything to do with hurried, badly designed, amazing-it-got-certified terminal software.