Well, you can't really be angry at them. They did warn us before the plot started that the prizes wouldn't be large, and that it wasn't a "real" plot. In fact, I'm pretty sure they were emphasizing those two things. That just happens to be the first definite reference I found, which was right before/when the Petpet Park mini-plot started (that issue was released September 5th).
I totally agree with this, the plot was very easy and they had specificaly stated the prizes would be small.
However, I had suspected we would be getting the new pet-pets (pets that would be common very soon, but since they're new it would be a fun prize).
Pet-pet-park items puts a bit of a twist on it. I'm not sure what that will actually mean though. They're a bit vague on what the park will become (other than the current collection of a few games), so I'm not sure what kinds of items will be used there.
Edit:
I admit I haven't looked at the
http://www.petpetpark.com/ site since it's in beta and I'm lazy. Perhaps if I had I would already know what petpet park items would be.