The reason for having better traction in the rear is that, in a panic stop, you do NOT want the rear end to swing out and spin the car. And in a panic stop, weight gets taken off the rear wheels, making them more likely to lose traction. So you want the rear tires to have as much good trread as possible.
The front tires, on the other hand, get weight transferred onto them in a panic stop, so even with less tread, the added weight improves the traction of the front tires.
The only case in which this logic is questionable is in the case of high speed driving in wet conditions, when hydroplaning could occur. In that case, the front end is more likely to hydroplane first, so you want good tread depth up front.
But, on the whole, the tire dealer is right.