That fine print has been on many offers for a long time. Generally, you received that offer based on your "historical" level of play anyway. If during that specific visit you don't maintain your regular level of play, or stiff the comping casino completely, you will likely find your next offers less generous or they will disappear all together. I've yet to ever hear of someone being charged at the end of a visit because of lower play - but it may come back to haunt you in the future by way of fewer or no offers from that property.
I'm sure someone will chime in with the example of the Venetian/Palazzo incidents where for a period of a few weeks a while back they were rescinding free room offers that people had booked, at the very last minute. That was a very short lived situation, and no longer occurs. It was simply a dumb knee jerk reaction by LV Sands Corp to poorer than expected earnings reports. They have since totally revamped their Comps program and they send out far fewer offers than they used to. From what I have personally seen, they now rely more much more heavily on the historical levels of play to make any comp offers in the first place and they no longer revoke any booked comps.