The reasoning for this is fairly clear according to the Mandriva CEO, who equated the deals to paying protection money:
In a statement on his company blog, Francois Bancilhon, CEO of Paris-based Mandriva, said, "We don't believe it is necessary for us to get protection from Microsoft to do our job, or to pay protection money to anyone."
Rejecting an offer is one thing, but when multiple vendors go so far as to pro-actively reject said offers before they ever come to the table demonstrates a clear distaste for them. The Mandriva CEO went on to elaborate on how he is not a fan of software patents or the existing patent system, which he refers to as counter productive.
The article goes on to cite other industry figures, such as a founder of the Open Source Initiative. Some are saying Microsoft is deliberately going after smaller companies or companies in financial trouble, knowing they'd be more likely to keel to their demands.
Obviously, many are resisting. This is a trend that many hope picks up, with other companies such as Canonical and Red Hat also saying no, sometimes multiple times. While many companies have joined with Microsoft, clearly the industry will not cater to the whims of the company which has so many times threatened it.