In a perfect world I would agree with you but gaming is a bloody expensive hobby so I totally understand why people get upset with short games.
That said, Ground Zeroes is pretty cheap and loaded with content. The story
is short, took me about 3 hours my first time, and it's a damn shame but there's much more to it than just the story. In contrast, Yaiba: Ninja Gaiden Z was released recently and I read that the game takes about 3.5 hours to get through and that's a full price title. Not sure how much replay value it has but I doubt it's even close to what Ground Zeroes has to offer. Comparing GZ with Yaiba may be a silly comparison to make but feel free to change it with any other short-length full-price game you want...
I'm horribly biased though so you don't have to believe me if you don't want to.