Exchanges have been caught commit a huge number of far more illegal activities, from selling user data to insider trading to helping themselves to users' funds for their own purposes. A little bit of fudging the numbers isn't going to phase them.
It's the way some operate their exchange business, if they don't add digits to their order books they don't attract investors. It's illegal but they've sold customers user data we know it.
USDT is insolvent and is not backed up 1-to-1 as they claim. This has been proven in court on more than one occasion. And yet it continues to be widely used. It makes no sense.
That's easy to understand USDT isn't safe so why don't traders see it? It doesn't make sense they're using it to trade USDT if it isn't backed 1-to-1 on the dollar. There is a risk trading USDT, perhaps it will bring Bitfinex down when investors make high speed sell offs.