There seems to be some confusion about commissionable and non-commissionable rates. In short, Marriott will usually pay out a commission to a travel agent/OTA/cash-back site for room rates which are high enough that there's money left on the table. If you're able to find a non-commissionable rate, it's usually a sign that you're getting one of the lowest rates available for that room on that night.

How do I know which rates are which? Easy, try to book a room. On the pre-confirmation screen, you'll see this:


Note the verbiage I highlighted. The rate I selected is a non-commissionable rate. For this instance, I looked under the hotel's Deals and Packages tab and found a Member Exclusive Rate (different from the normal Marriott Rewards rates, which ARE commissionable). In this case, the regular rate is $211/nt, the Member Rate is $201/nt. Even with getting 10% from a cash back website, it makes sense to book the non-commissionable rate at $169/nt instead of the commissionable Member Rate at $201/nt (effectively $180.90/nt w/10% cash back).

If it doesn't show that verbiage at the review screen, it *is* a commissionable rate, and cash back websites will work.