Because you can't make all drop-downs work that way, and the user shouldn't have to guess how it works. For example, when filling out your address, the STATE may be a drop-down. Or for setting scope for a search. The search won't happen until you click the button.

Now if there's a message that says something like "choose a destination" (or whatever), then fine. Or if it's an intRAnet, make your own rules. But if someone's not paying attention and the page changes and they don't realize it, it's trouble.

I've even run into this when I move the wheel on my mouse when I happen to be in one of these "touchy" selects. The page goes crazy :)

Tastes vary.