I don't think a disc manufacturer can force a player to cancel or turn down a bag contract with a non-disc-manufacturer company. If all they do is bags (or apparel), there's not really a conflict of interest. What the disc manufacturers can do, however, is incentivize carrying their bag enough so that it's a better deal than going with the outside company.
A disc company can do whatever they want with a contract. What they choose to do is a different story.
A discraft player told me he could use any bag, so long as it wasn't from another disc manufacturer, Lat, Gateway, etc.