Is this Japan-centered? Because most PC games have demos nowadays. If not demos, betas, to showcase the engine before people play it on the actual game. Console games have quite alot of demos out too. I count alot of racing games, sports games, RTS... even a few simulators. What's wrong with putting out a fighting game demo? For example, all XBLA games are obligated to have a trial game available for download, with just a tiny bit of the game to show off before buying. It's a way to show people how the game is. It's not just for kids, most adults would like to try out a game before buying.
It's not really a Japan-centered thing at all. It's more of a thing that the Fighting Game market is, again, quite a ways different from everything else. Capcom and Namco, the two "leaders" in the genre, know this much to be true. Furthermore, I believe the individual fall-outs that resulted from both HDR and MvC2's demos have only done more to showcase why there is such a reluctance to do such a thing.
I mean, Capcom just put out a demo for Sengoku BASARA 3 on PSN a day or so ago. And that applies only in Japan, where it stands to benefit those fangirls and kids who want to try out the game before they buy the full game tomorrow/today. But unless I stand to be surprised, MvC3, for example, won't get such a demo. Thus far, they've been doing the exact same thing with the game as they did with SF4 years prior, where the game's progressive builds were passed along at live demonstrations/loke tests instead of thrown up on PSN/Live.