Steam is required for Half Life 2, even if it is just for activation. Therefore it's a good idea for them to require Steam for the demo as it lets the user know everything they need to be able to play HL2. Also, if they didnt previously have Steam, they will now and they may purchase it over Steam rather than a retail copy. It's just smart business.
Its called a readme file moron! Its called reading the system requirements BEFORE you download. If you can't figure that out, do us all a favor and sell your fucking computer to someone who has the synapses to use it!
purpose of a demo is to get as many people as possible to play it in order to show it off so they buy it....getting steam is a stumbling block to that = complete lunacy.