They didn't have the foresight to upgrade/update their network to handle the large data load, and for a business, it's much better to make more money by charging your customer more for going over their data limit.
Also, I'm grandfathered in and I have the 5GB throttle down. I'm not sure how the OP managed to not get affected by their mandated limit. It actually should say it in the contract that they'll throttle you down. They discussed it in length with me when I was upgrading to the Note II because I questioned it.
Personally, I don't like having a limit hanging over my head before I'm throttled. The reason I chose unlimited from the very beginning is the convenience that I don't have to worry about going over some data limit and suffering consequences. And believe me, with all the things you can do with the Note II, it's not that hard to hit that limit, accidental or otherwise.