I've never experienced that. I find that they generally stand back to give you some privacy while you enter the tip and pin. However, they usually feel the need to make small talk while you are completing the transaction. Drives me crazy when they ask what my plans are for the day, as if it's any of their business. I have seen a few places that will leave the handheld machine while they check on other tables, but not many.

Back on topic, a default tip of 25% is ridiculous. One local restaurant uses a machine that equates each tip with a level of service, to make you feel guilty for choosing a lower tip option. It actually gives you choices like "15% = adequate service, 18% = good service, 25% = great service".