It's more like the other way around, where the device is tied to the ID until you change it and whatnot. You can use the same ID on multiple devices (common for owners of both iPads and iPhones). I would say because the iPads tend not to have that much personal information such as contacts and calender events and such and such, there'd be a slightly more open access to it. Don't quote me on that particularly, just speculating.
Phones are generally the "holy grail" of information. If you have a phone that someone uses frequently, you can tell a lot about the person if you gain access to it, which might be why the iPhone has stricter security than the iPad. in my opinion.