In a standard commercial situation, the buyer gets a product in return; in a casino, he only gets a slim change of winning something, and more often than not, doesn't.
Ahh but you're forgetting, the 'buyer' if you will, does get something in return at a casino. Say you play roulette. You put $200 on Red. In return you get a few things:
1.) you get a 47.4% chance of winning $200
2.) you get the thrill of gambling and the excitement of chance
3.) in most casinos, you get free booze.
Now, clearly there is a measurable value in #1, and depending on your drinking habits, #3 as well. I caution you, however, not to devalue the second one too much. All the time people will pay for entertainment alone. You go to a movie, you pay to go rock climbing, pay to play mini-golf, or regular golf for that matter, you go to an arcade, all of these activities require the buyer to pay money without getting anything tangible in return, but no one ever says that the $0.50 it costs to play street fighter is 'cheating' you out of your money. You're paying for something.
It's actually a fascinating concept though. If you are up front told that it is impossible for you to win anything and you are paying to play for fun only, then we feel 100% comfortable throwing our money away for the sake of fun. The second the prospect of winning a prize for your efforts is introduced, we feel ripped off if we don't get a prize. Even if the game itself still provides the same level of entertainment.
A classic example is the Crane Game. If you KNEW that you didn't actually get to take home the stuffed animal if you captured it, but that you would simply be presented with a screen saying "YOU WIN!" I assure you people would still play, because it's a game of strategy and dexterity and skill whatever. The second you allow them they plausibility for a prize, if they do not earn the prize, it's all "TWO BUCKS FOR THIS!? WHAT A RIP OFF!!"