On a more serious note, there are edge cases such as the one I sited in a reply above. In addition to being a POS terminal, it functions as a web based lookup terminal for something else. Also, depending on the application, the POS terminals phone home the sales numbers for inventory purposes. Yes, it probably would be better done with a dedicated modem line, but that would probably just lead to a different hacking scenario.
But yes, for most instances you shouldn't. The thing is, today a cheap PC with POS software probably costs less than a dedicated POS terminal. So that's what you get. Since the PC comes with the browser, that's just a "bonus".