I'm guessing that a big part of the reason of the carrier's pricing & locking system is their financial calculation.

They've made their calculations before investing in all the infrastructure with certain assumptions, and those assumptions include components such as:
- number of customers per base station
- customer growth\cycles
- expected income from each customer

You can see how an open system that brings free\low cost alternative solutions to what they're selling (voip, messaging system, etc) may mess up with with their calculation and they're fighting it off to delay the inevitable.

You can be sure that such calculations made nowadays already take into account disruptive devices such as the N900 (for any new infrastructure investments), but we just can't expect things to change overnight.