Some of it can be chalked up to "astroturfing" and viral marketing (I suspect a fair bit of it, in fact), and from people who bought the anti-Nintendo spiel hook line and sinker. You don't hear a lot of genuine anti-Nintendo testimonials; most just rely on tired cliches and generalizations, without any sort of meaningful observations about the system that are genuinely damning.
To wit: for all the anti-Wii-Sports posts out there, not even a meaningful fraction of them bring up genuine issues with the control schemes. You'll see waves and waves of generalized "this game sucks" posts, but nobody seems to say anything detailed like "the controls in the Baseball game are very picky, and on the off chance you do land a hit, you're at the mercy of the game's randomizer to decide if your hit results in anything ranging from an immediate out to a home run". Suggesting that most of the complaints are from people who actually haven't played the game, or if they have, haven't played the game long enough to come up with actual issues.
In short, anti-Nintendo sentiment appears to be by and large from people with little to no actual experience with what they're complaining about.