It's often done to make support calls easier. You could spend a long time trying to explain to a frustrated customer how to use telnet (Type a slash... no, that's backslash. No don't type the word "slash"...) Or you could just say "let me log onto the router and fix it for you."
Telnet wouldn't help if they forgot the password and it wasn't hard-coded as a back door. Also, the support tech wouldn't be able to access it remotely unless some of the things I mentioned in the original comment were true.
Wouldn't it be easier to just say, "hold down the reset button on the back for 30 seconds"?
