Casinos really took off when /warp casino was added with tutorials on how to build them.

If you look at the prize wall at "TheCasino's" Island and the chances of winning those prizes, apply a little maths and you will see that that one casino alone has raked in around 260,000 grass. Accounting for what they would have spent on buying the rares in the first place, it's still a profit of 130,000 grass. Average profit on those rares is about 800 grass each!!!

Even if the majority of casinos are accepting DP heads for spins, there are still plenty enough accepting grass and pulling huge amounts out of the economy. Like yours emil733 .
Maybe those accepting DP heads are doing so because of the grass shortage.