Yes, its the developer's fault that you pirate the game.
Honestly officer, he was right in the middle of the road if he wasn't there I wouldn't have been able to drive into him!
(Yeah I know that's not what you are getting at but your reasoning still has a bad core notion.)
Someone who pirates a game and then buys it does not represent a lost sale. Developer's don't like it but so long as you are paying in the end its not the end of the world. Its the numerous people who pirate a game and don't buy it that are the problem. Assuming that everyone who pirates it for a demo will either not play anything more (still a problem for the same reason the grocery store doesn't like you "trying out" there grapes before you leave) or will buy it, these pirates are not a financially taxing issue and thus not worth the developer's time when they could be targeting the assholes that actually do download the game and never pay anything. Demos take time and money away from the main project and your own reasoning is that it would simply make some people who pirate as a demo happy while the real assholes wouldn't care. It's not in there best interests even if it does so what you are proposing.
Aside from that. You have the god damn internet. The greatest source of information known to man and one that catalogs every imaginable thing. You can get an idea of if you will like a game if you look hard enough, I guarantee it. Assuming you're looking at something with any sort of popularity it has information, and even if it doesn't there are numerous websites that will get information on it if you were really so inclined to try. So long as you are posting on an internet website saying that you can't get information is an utterly lousy excuse.