Yes its only the breeder that can register them. Just because their pure bred doesn't always mean that they can be registered either. If the breeder intended to sell them for breeding they'd of registered them, they might not think their good enough, or if they were intended for breeding might of been more expensive. Or the breeder might of brought pedigree stock that weren't registered and bred them, so they couldn'tbe registered?
I'd speak to the person you brought them off and ask if they would be willing or are able to register them.
If a breeder thinks a pig is eligible for registration they wont necessarily have registered them as weaners. It is a two stage process. They must be birth notified and then BPA pigs can be registered by the owner at any time in their life. British Lops must be birth notified and if they are to be registered they should be done before they are one year of age. So, you may well have bought birth notified weaners, suitable for registration and now the breeder needs a prompt.
Meat pigs can be birth notified, they are then traceable and can be sold as pedigree meat.
As bj_cardiff says pure bred and registered is not the same thing. Stock can only be registered if both parents are registered. Breeding stock must be identifiable by either a notch or tattoo (or double tagged if the breeder has that arrangement with the BPA).
If you bought meat weaners then the breeder will not and should not register them. If you bought stock for breeding and he sold you pedigree stock then it should be a simple process to sort out registration. As bj_cardiff says the price should have reflected what you bought.