I'm sure someone mentioned something before that the pacers had something to do with local councils who had paid for them so whoever takes on the franchise has to take them on. Not certain on this though so don't hold me to it. Would make sense though as none of the leasing companies have them on their books
Matt is correct. The units in question were purchased by Local Authorities. It is part of the franchise obligation to transfer 'ownership' to the current franchise holder.
It could however, be renegotiated in the future under the terms of a new franchise. Local Authorities could be pursuaded to do anything if there is a positive trade off elsewhere.