I don't think it is really as malicious as that. MS gives special rates to large manufacturers and part of getting that special rate is exclusivity. If they gave a company a special rate and they break the terms of that agreement I don't think it is malicious to pull that rate, or at least warn of that possibility. Companies should be allowed to use special rates and other incentives to encourage the use of thier product, and revoke said incentives if necissary.
US anti-trust law says different, and MS didn't dare face off with the US DOJ over it (entered a consent decree in 1995 where they guaranteed they would end this and other anti-competitive practices), because it is highly anticompetitive for a company with a near monopoly to use that position to strongarm customers into not dealing with their competitors.
EDIT: You might also note that MS was later sued and *convicted* of a spate of violations of US anti-trust law. In fact, had it not been for incompetence on the part of the judge (spouting of to the press), MS might have been broken up into pieces.
