I always just tell them "to use on a different carrier." I don't think they care, they probably just have a field to fill out in their system. if you ask them to do the unlock request for you I'm pretty sure they just fill out the online form.
Thinking about it some more, if you lie about it and say something like "to use overseas" when you actually just want to use it on tmobile, that probably hurts us more than anything. If they're collecting the data and they see "Oh, a lot of people want to take their phones to a different carrier" it would probably help in getting better pricing and options. As opposed to "because I want to" which they can't really respond to.
The solution is very simple, by law no locked phone. like some countries do.
you want to move to a deferent company? good pay of your phone or the new company will have to take over the charge of the phone, and it easy to force,
by the IMEI #.