As part of the hotel contract with Marriott they are to allocate a certain number of free nights rewards per night. Points operate a little differently where points can be used like money.
For those free night certificates the hotels turn them into corporate and get a certain predefined set base rate for the hotel rooms.
The points used likely has a different value for redemption depending on how a hotel adjusts their category level based on the season and time of year.
Similarly based on these peak and off seasons the hotel can limit the amount of free nights that are available.
I did this with Best Western and got to talking to the manager and they told me with Best Western they are given a contractual rate for the hotel stay. that rate was a reasonable comparison to the regular rate. High demand seasons they can limit point/certificate availability than in low demand seasons or low demand nights.
There is a hotel near where I live that puts a minimum require of nights stays for weekends. They possible can limit for weekend stays saying those certificates cant be used.
The changes in the program now turning these free nights for a year is much better than in the past. What I personally got to hate was those cash back promotions that operated as a coupon for future stays. There were too many hidden restrictions in it such as the $100 certificate can only be used for a 2 night stay or it can only be used when booking under a different price point. An example was with Carlson Country Suite Promo going on now where you stay 2 consecutive nights you get 25,000 pt bonus....the catch was the price point rate was higher by about $25 per night than the best available or AAA rate.
SUGGESTION: Marriott should allow a point + free night certificate for higher level stays. Either allow the certificate to be turning into points (maybve 10,000-15,000 pts per certificate based on status--silver get 10K, gold get 15K, and platnum get 20K or a 10K/12.5K/15K btreakdown)....or allow the certificate + 5,000pts= cat 5, certificate= 10,000=cat 6. You can attention points to expiration dates. Airlines do that with miles.