People aren't assuming they are engaging in fraudulent behavior - there is evidence showing that they are. All they have to do to show they aren't is provide an audit.
> “My personal biases make me inclined to believe that Bitfinex is not using Tether to manipulate the price of Bitcoin,” said Jeremy Gardner, a managing partner at the investment firm Ausum Ventures. “If they have attempted to do so, which would be deeply concerning, it’s hard to imagine that such fraudulent issuance at its current volume could single-handedly buoy the price of Bitcoin.”
My point is that being skeptical about their ability to manipulate the market in a meaningful way and make a profit doesn't mean they don't have a profitable motivation for engaging in fraud.