Upload & Sell: Off
Jeremy has it right, I believe. If you're in a town/area where USATSI already has plenty of photographers, you're probably not going to hear from them for a while, if ever. However, if you're somewhere there is a need, you'll find they're more responsive. In a perfect world, all emails would be answered, but how many of us believe the world we live in is perfect?
Also, in larger areas, where there are many photographers working as stringers for several agencies, when there is a new opening (somebody moves out of the area; gets too busy with other things; gets out of photography; etc.), it's very possible that USATSI will attempt to fill that void with a photographer who is already known, from the local roster of qualified shooters.
Go on the USATSI web site and do a search for your town or region. Take note of how many photographers there are, and how often each of them seems to be shooting. As a reference, in my kinda town, USATSI has three primary photographers, one or two folks who are used to fill in the holes (who also shoot for other wires), and the aforementioned Director of Photography who also occasionally shoots. This is a big media city: we have 2 MLB teams, an NHL team, an NBA squad, an NFL team, an MLS franchise, one from the WNBA and 2 Div 1 NCAA schools - and we cover pretty much all events with only 3 photographers.
Hope this helps. Don't get discouraged.