They charge you because Fido (and all service providers in North AMerica, where consumers are tight assed cheapos) heavily subsidized the price of the phone, and doesn't see any reason why it should sink several $hundred into the cost of your phone just so that you can take it and pay someone else for the service.
The $200 is probably the remaining cost of the handset that they have already subsidized. They're basically asking you to finish paying for the phone if you want to use it with someone else.
If you'd like to get an unlocked phone, you will quickly notice that they are a lot pricier than anything you paid for at Fido (or anywhere else)
-jM
A&D