I don't know how you get the offers that you do, but the Riviera and Binion's offers had the following wording "free play valid with any hotel stay".
The Fremont offer was "free room up to four nights....$50 in slot dollars - Must be staying at Fremont to utilize this offer".
I think since we live within an easy drive into Vegas, they want to "hold us captive" at the hotel to try and get more $$ out of us in the long run (silly rabbits! )