While I agree with the "it is their game" statement, I actually don't think Marriott forbids anyone directly from using any rate. It is up to each hotel to decide who they want or don't want to allow for any rate that they are using.

I have used both the Travel Industry and the Gov't rates before even when the T&Cs said something like "only airline employees" or "only Fed employees" (I was technically neither though I was working as a vendor for each). But in all those cases, either I (or someone at the client I was visiting would contact the local hotel and work out our use of that particular rate.) As long as the hotel management says "we will allow it", we are good to go. Marriott doesn't really care.