Who said it was? And what's wrong with having a standalone CC terminal? That's how most very small businesses (food delivery, auto service, tables at shows/markets) still take credit cards. Their wireless terminals aren't "integrated" into anything else, it's just a swiper/PIN pad with a 3G modem, and Square is the modern version of that.

It's certainly prefereble to have your POS integrated with your payments system. Otherwise, there's no guarantee, for example, that the amount charged matches the POS total. In addition, it's much easier for employees to steal, and it simply takes longer.