Well, can't really fault you there. If all I did was play the single player campaign for SC1 and 2, I'd probably feel like I was getting ripped off too.
I still don't see the complaint about the single player. I thought the campaign was brilliantly designed, and it's only short if you play it on too easy of a difficulty setting. If you are a veteran of SC1, or a good RTS player, then of course "normal" will be pretty quick and easy. Getting all of the achievements in the campaign is quite fun as well (which you can't do on just normal difficulty). Then there are tons of single player "challenge" missions which are quite good as well. Not too mention the really fun "lost viking" mini game hidden in the single player.
Honestly, the single player is just as long as SC1, but is better designed, and has way more replay value. There are even a few missions that you can't get it in just one play-through, or missions that change dramatically based on earlier choices. So with the different difficulty levels, upgrade choices, alternate missions, hidden mini-games, fun achievements, and challenge missions, how is it possible to call it only 1/3 of a campaign and a disappointment? Go back and play SC1 and Brood War, then play SC2 and give me an honest evaluation of the length, depth, and quality of the campaign.