I know people who feel pressure to work on vacation. The pressure is very real and isn't always related to ambition on the part of those people. Usually they're being exploited by an ambitious manager who has a "hold" of some kind, e.g. exploited person has a child with 2 years left of college and can't afford to lose his/her job.
One Seattle-based eCommerce company forced you to keep pager with you all the times, even on a vacation, if you were an unlucky part of operations.
This can't happen in most European countries, unless the company desperately wants to lose plenty of court cases.
One member of my US family in the midwest worked in an unionized menial job with 1 week of paid vacation including sick days. The rest was unpaid.
Only after I left the unit I was in, was it respected that I was not to be contacted.
Edit: I'd also add I had to work all day on Christmas one year, and I know others in very similar situations.
Friends with a security engineer at a company everyone would know. He can't even get a normal work day right now, or a single day off in a week.
It certainly happens.