I think I understand the difference. Mobipocket can sell books anywhere because they are not the exclusive publisher of their format in any market (besides possibly France). Any one can publish in their format. Fictionwise, for example, relies on Mibipocket DRM servers, but likely negotiated not with Mobipocket but with the various publishers, agents, authors for the right to sell in the US market.
Sony, on the other hand, is the exclusive publisher of its' format.