I think people are free to do what they want with their money.
Uh, yeah, they are, no dispute there. But when they come to the MMM forums
asking for advice, I assume they want Mustachian advice - so that is what I gave. My mustachian advice is that by changing his mindset, he can get the same entertainment out of it without actually gambling: after selling all his stocks, he can be entertained if the stock does badly, instead of if it does well.
If it brings you $100 of enjoyment then why not do it?
Economics. In a competitive market, the price of goods and services is not determined by their value to the consumer, but their (marginal) cost of production. When you say "if you get $100 of enjoyment out of it, fine", you're talking about the value it has to him - but that is not what determines the price he should pay. With an appropriate mindset, the cost of enjoying watching a stock rise or fall is $0, and a frugal person shouldn't pay $100 for entertainment that he can get for $0.
If somebody was paying $100 for on-demand movies through their cable TV, would we say "well, if you get $100 worth of enjoyment from it, fine"? No, we'd tell them they can get the same enjoyment a lot cheaper from other movie services.