Could also be a leftover computer glitch that Marriott has been trying to work out for some time now.

I had just the opposite problem this weekend....was trying to book a paid stay and the award rooms kept showing up.

Best way I found around this was to start over completely, either going in under the brand name (Courtyard, Renaissance, etc.) or by clicking on the actual hotel name on the hotel's main page (the name with the black line under it) and resubmitting all values.

Don't give up!