Well, I guess it kind of makes sense. If their goal was to make it so he didn't get out of projectile oki, and their solution to that was to make the counter blockable, then it wouldn't make sense for it to be + on block. After all, then Haku would be the one who could make the next move and basically get out. If it was minus on block, then you could block the counter and then keep applying pressure.
It's probably like -3 or something though, something that isn't punishable but that still lets you get the first hit in if both of you try to hit buttons right away. All speculation though. And of course you could just counter again.
Then again, you could look at it the other way too. Not getting hit by the counter and being able to get in close and block it might be enough of a reward to justify letting it stay positive on block.