It's pretty common, actually. They do it for the reasons awa pointed out, as well as to reduce the debt burden for the compan. IANAC [1], but I believe this has something to do with unused vacation leave being charged as outstanding debt. It's also a way of saving money by shutting down operations at a time when less gets done anyway.
Sun, like most companies, did give the option of taking the leave without pay and preserving your vacation time.
Maybe that's one option for US companies - pay for two weeks of vacation time, but culturally allow up to six without pay. Then we'd actually be somewhat more like European countries, but with an option - reduce your salary if you'd like more vacation, but keep working and make more money if that's more your style.
I doubt I'll see this happen - first, "leave without pay" is unusual in the US, typically done medical conditions where the employee wants to return to the job, but will be forced to leave for long periods of time (pregnancy leave is often handled this way). Another problem is that it may actually be inefficient - it would create a financial incentive to take minimum vacation time, and a lot of people believe, with some evidence, that workers are less productive this way.
[1] IANAAC: I am not an accountant ;)