Thanks for the replies, which has certainly enabled me to put my complaint "in context." It looks like a classic "gotcha" for which there isn't a clear remedy. On one hand, assessing a $46 service fee on a "free night stay" certificate is pretty unconscionable; on the other hand, having this deluxe property classified as only a Cat. 4 is pretty favorable.
I have dashed off an email to Marriott customer service noting my belief that a "free night" should mean exactly that -- and pointing out that indeed it does at competing hotel chains. I'm not expecting much in response, but I will post what they say.