why there's so much negativity out there from some people about steemit
I don't know anything for sure, and these opinions are my own. But here's my best guess:
If you get psychologically invested in a tribal identity as a "Bitcoiner," you'll start to have feelings about how all other tribes are stupid, smelly, inferior barbarians. The same thing happens with any cultural identity, really. Having feelings like that isn't within your control -- but how you allow yourself to react to them certainly is.
Some people will react to the feelings generated by their tribal identity as a "Bitcoiner" by reaching for arguments that confirm and legitimize their feelings. No one will take you seriously if you say "Steem is a bunch of stupid smelly barbarians," but they will listen if you say "Steem is a scam because the economics are fundamentally flawed, here's why supply demand capital flux capacitor et cetera et cetera," especially if they also identify with the same tribe as you do so this argument points to a conclusion that agrees with their tribal-identity-based feelings.
Of course, a lot depends on your definitions. The basic idea of Steem is that by buying Steem and powering it up to get Steem Power, you'll have much greater impact on the visibility and monetary rewards of posts you upvote. If we define a "scam" as "not getting what you paid for," then Steem clearly doesn't fit this definition of a "scam." Reputable exchanges  will give you real Steem for your Bitcoins (or vice versa), the Steem really can be converted to Steem Power, the Steem Power really does affect the visibility and monetary rewards of posts you upvote, and on the whole the system does function more-or-less as described in the whitepaper and official posts.
But it seems that some of our detractors are defining as a "scam" any scheme in which an asset's value depends on the availability of future market participants willing to buy that asset. By this metric, almost any imaginable token, currency, or good which can be resold is a "scam", because its value depends on people being willing to buy it in the future! This is an incredibly broad, useless, and non-sensical definition of "scam".
Whether the Steem economy is sustainable over the long term is something which depends on many factors. Even assuming the current economic model of Steem is unsustainable , that does not mean that it will not become sustainable in the future, nor that buying Steem is throwing away money. When Facebook and Youtube started out in the last decade -- or for that matter, Google and Yahoo in the decade before that -- a lot of critics mocked and derided them as economically unsustainable ventures. But that didn't stop investors from putting money into them which subsequently reaped enormous gains, nor did it stop those businesses from eventually finding and developing revenue sources to justify their valuations.
 Whether a particular exchange is a scam is a separate question from whether Steem is a scam.
 A questionable assumption, but for the moment let's accept it for the sake of argument.