I've worked at two startups with no vacation policy. My inference is that it is very much a cultural thing. My first company some people really took the opportunity to heart but most were subtly pressured not to take much time off. Plus, lots of people did working vacations. Then at the beginning of September last year they told the entire marketing team that there was no more vacation until the end of the year because the CEO felt marketing was gone too much that summer. In fairness, we had Christmas to NYE off but still. Plus, it made it harder for the sales team where they always had to meet their quota because vacation was never factored in.
In my second company, people take what they need and my boss told me not to contact him unless it was an emergency. I loved it. Set the cultural cue for me on how he perceived vacation time. People seem to take what they need. The CEO encouraged people to take advantage of our ski passes this past season during snow storms.
Bottom line: Management sets the cues and cultural norms. People practice what they see when there aren't official guidelines.