A good point Lunchy, and one that could be addressed by riders/promoters signing a contract which allows for compensation to either party in the event of a breach (and I'm assuming that current contracts aren't worded in this way).
If a promoter sees fit for business reasons to replace one of his riders, he may do so provided he finds the rider alternative employment (a place in another team) or financially compensates him for loss of earnings etc.
Conversely, if a rider sees fit for personal reasons to ask for a move or withhold his services during the season, the promoter could then seek compensation to allow him to source a replacement.
Seems logical to me - any thoughts?