There is a bit of debate over if it's a bug in safari or not for safari to allow you to tab navigate to dropdown menus.
I'll include some select quotes:
(In reply to comment #6)
> (In reply to comment #5)
> > It's interesting (and perhaps a Safari bug) that Safari does this, because
> > popup menus elsewhere in the OS definitely don't get focused when FKA is off.
> Apple seems not to think it's a bug, since Safari 3 (along with all WebKit
> browsers) continues this practice. I still think it's odd, though, since it
> doesn't match the rest of the OS. That, or hyatt hasn't figured out how to
> differentiate between select-as-popup-menu and select-as-listbox yet, which I
> find improbable ;)
So, should Safari do this, or is it a bug?
It's not a bug. We do this on purpose for compatibility.
We make an exception for the dropdown list even though it does not match the OS, since it just makes it easier to fill out forms from the keyboard without having to turn on full keyboard access.