Hmm -- interesting question. My corporate travel site forces you to check the "I have read and understand the rate rules" box before proceeding, which I have once or twice, but the marriott site has no such requirement. The rules are printed on your reservation statment and are available via a link when the rates are shown - I'll leave it to the lawyers/Marriott reps to define what is "required" on a user's part.

I know that ignorance is not a valid defense in the legal arena, but this seems a bit fuzzy.

I certainly agree that Marriott screwed up making a rate available that violates the rules for that rate, and I can't imagine them trying to enforce it (unless, of course, you post all over the web that you know in advance your reservation violates the rules )