Why doesn't Apple subsidize the cost of the POS devices that are compatible with Apple Pay? If the big hurdle for businesses, big and small, is the cost of upgrading their POS, then perhaps a company with $200B in cash sitting around could help pay for that. Average POS device is $2000 (1), and there are 10 million POS devices (2) in the US. If Apple covers 50% of everyone's upgrade, that's $10B. About 3 months of profit for Apple.

1 - http://smallbusiness.costhelper.com/point-of-sale.html 2 - http://www.gaoresearch.com/POS/pos.php