I believe that Paul has always stated that he can use any disc he wants. And I imagine that it is their contracts that way. However if you work for a company your pretty much expected to use that company's stuff or at least their partners and subsidiaries. However if you start tossing discs from a competitor i doubt that your company will keep you around very long.
A couple of examples: I used to work for a University as an employee of the i was expected not to wear clothing featuring other Universities... (say you work at Duke, and wear UNC gear to work) Though not specifically against the rules it was very much frowned upon and while wouldn't get you fired it would hold you back. Another example i can think of is if you worked for Coca-cola and came in every day drinking Pepsi, I imagine you wouldn't be around long. Brand loyalty is a pretty substantial thing for employees of a company.
But since i am not an employee of a disc manufacturer i throw a mixed bag.
1 Axiom, 1 MVP, 2 Lat 64, 2 Westside, 4 Innova, 2 Discmania