I did a LOT of POS work a few years ago for small businesses. An iPad/iPhone would be dead and broken in less than a month for most retailers. You need pretty hard wearing industrial grade kit if you don't want to throw it away 3 times a year. That flimsy card reader is almost comedic - you'll break it and your device.
If it's your revenue collection equipment, it makes sense to buy something that will last i.e. was designed for the job rather than sit there not being able to take cards.