Banning is a rule against it, and won't stop poaching. The idea behind this ban is to make us not want to use ivory. If there is less demand, there will be less of an incentive to risk getting shot. Poachers already face that danger, and short of drone strikes it would be very hard to make poaching riskier. Perhaps after the demand for ivory has gone down, we can use the "good" ivory. Until then, however, the more ivory we use, be it poached, antique, or legally harvested we are creating incentives to poach.
But, as mentioned in this thread, poaching isn't the biggest problem elephants face. Also, the US is not the only consumer of ivory, so this ban is more anti-poacher than it is pro-elephant.
Finally, how do you know that all of that 100 tons isn't poached? If somebody risks getting shot for some elephant tusks, I'm pretty sure they would have ways of leaking that ivory into the market.