It's anywhere close to open and requires vendors to seek approval from Apple for every implementation - Apple just has the market share to make everyone dance to their tune.
Apple Pay works on any terminal that supports NFC tap to pay payments. There are thousands, tens of thousands of terminals in the US that are not "Apple Pay" that you can use it on
Same here in australia. We had NFC payments ("tap and pay") since ~2006. Nearly a decade before apple pay launched in Nov 2015. For us, apple pay was like "oh, I can use my phone instead of my credit card? Neat."
People use it everywhere here, just because its easier than carrying around a card.