I've been using the site for a couple years or so and I've always noticed that literally every deal listed ONLY has positive votes. And if someone does neg a deal for a legitimate reason, other members seem to take offence and neg that person in the comment section, resulting in the neg on the original deal being revoked.
So whats the point of having the + and - on deals, if the community isn't welcome to neg deals??
The deals that have an overwhelmingly negative response tend to be legitimately bad deals and are removed from the page, and become invisible to us.
That's why it seems as if deals with negative votes don't exist. They do, they are just deleted pretty quickly so you never see them. Edit: see Neil's stats
Remember whether a deal is good or bad depends on the community's response, it's not down to exact science or mathematics — if 100 people think a deal is good, but 2 people disagree and click the red button, a large majority could still disagree and downvote the two into oblivion. Most just neg vote to prove a personal agenda and aren't contributing much to the discussion at hand, and these are more likely to get slammed.
Is this a 'flaw' or an intended feature of ozbargain? Who knows, but the entire purpose the voting system exists is to give pretty much everyone a say, whether you have approvals or misgivings about the comment itself.