> If that is the logic, why make jailbreaking difficult at all?
Apple pays support costs (facilities, labor, etc.) for everyone who brings in an iOS device to a store, and it's their best interest to allow their personnel to turn problems around quickly. If an in-warranty device is "broken," they can send the customer home with a refurb one within fifteen minutes (I've done this for a screen). An easy jailbreak means more devices will come in broken, they'll have to keep more replacements in stock, and they won't necessarily be able to turn customers away for it because there might not be a good way to detect a jailbreak.
Jailbreaking is also a security problem for enterprises. If a user can run arbitrary un-sandboxed code, there's no way the device's owner can trust it to receive sensitive emails, be wiped remotely, and so on.