iPhone just wasn't going to exit the curious gadget status and become a first class citizen in the Japanese phone market without it. Same for emoji support and flick inputs and lots of those little things.
They were never forced to compete in the sense that no one is forced to be first class citizen anywhere. Apple just wanted to be by its own free will. That's technically correct.
But that part is on Apple. They could have made it Japan SKU exclusive. It isn't solely because it's their policy that an iPhone is universally an iPhone and there will never ever be Verizon backplates or brown box CostCo specials with plastic backplates. No one else cares, so everyone else make FeliCa enabled versions as Japan specials.
They were never forced to compete in the sense that no one is forced to be first class citizen anywhere. Apple just wanted to be by its own free will. That's technically correct.