Or at least often not by law, there are some stupid laws around WiFi/broadband etc. which can be interpreted to state that it's not allowed for a phone to be sold which can be rooted (without a hack) as the user could use it to setup a WiFi hot-spot which uses non-legal frequencies. This law was made because supposedly that (with routers) is a problem, except it isn't as far as I know and it as pure lobby work from a certain industry which also loves the user to be forced to use their routers.

(PS: Also country dependent.)