Yeah, and most companies only let you accrue a certain number of hours of time off, typically somewhere between 100-200 hours from what I've seen. Once you hit that limit you simply don't get any more time off. Your boss won't let you take a vacation until next month? Too bad, sucks to be you.
Thankfully the way my current company handles this is about as good as you can get - they cash out balances that are too high a couple times a year at prespecified dates.
This must be a California thing. It's not generally true in most states.
And in a lot of companies, even when they specify an amount, it is a "recommended" amount. My company, for example, says 3 weeks, and many managers follow this religiously. But when you read the actual rules, it says that 3 weeks is the guideline, and not a promise. This is also used as an argument against giving 4 weeks: "3 weeks is just a guideline. You can work out more if the manager agrees."