Do you have an email confirmation that showed your original details? Fight it w/ that. Get on the horn immediately (given the charge or cancel tomorrow).
I'm assuming you're referring to CES, which gets 175,000 attendees. You might have made a ressie before all the T&Cs kicked in for CES, but that's not your problem. However, it explains why they're enforcing subsequent ressie T&Cs.
I'm assuming, since there's now a 'destination amenity fee' that you're at the Renaissance near the South Hall. It just instituted one. The other Marriott brands (including the LAS Marriott) around the LVCC haven't done so).
If you don't have a printout/email confirmation, get on the phone asap w/ Marriott cust service & have them pull up your original ressie to show the date it was made & then point out you shouldn't get charged for the entire cost etc.
Fight back hard on this one.
Cheers.