the reason because one has the activate the phone at the store

The iPhones you can get on eBay have been limited to first-genration units (for obvious reasons, though that may change once people start buying the second generation just to cancel their contracts and unlock them), and many people either don't want a first-gen or aren't comfortable paying well upwards of $500 for it.

Also you have to use the phone with rogers and fido anyways so it doesn't make a diffrence in terms of the data rate pricing