As of today, and forever forward, Movable Type is open source. This means you can freely modify, redistribute, and use Movable Type for any purpose you choose.
This is a good thing. MT had previously had a policy that you could make changes to the code but you couldn’t redistribute those changes. MT has an architecture that supports plugins (optional add-ons that change the way MT works, but without messing under the hood). Several times MT tech support people have recommended particular plugins to me, and several times free third-party plugins that were very popular were integrated into subsequent versions of MT.
I paid a reasonable fee (a couple hundred bucks) to license MT3 for blogs.setonhill.edu, and the past few years I’ve paid another reasonable fee for technical support that has saved me hours and hours of frustration.