I totally agree. People booking on the internet aren't required to read the rate rules to ensure that their reservations obey all the rules. If the system accepts it, that's all I need to know! I've never had a problem.
If Marriott -- or another hotel chain -- wants to limit the nights that a rate is valid, then they shouldn't load that rate into the system on ineligible nights. If they do their job, I won't have to do it for them!
Bruce