They're hosted by Network Solutions LLC. As a result, the host should be fairly responsible for enforcing US laws on the websites which they host.
Though freeware, you still own the distribution rights. They do not have the rights to copy or distribute your work. They don't have the rights to upload it.
If I were you, contact their web host and let them know that this website is taking your software and redistributing it.
It may seem like a far measure, but the host is responsible for the content they're allowing to be used. If FSPlanet won't listen, the host probably will. If you get yourself and your fellow designers to complain, I'm sure something will be done.
"FSPLANET.COM - THE WORLD'S BIGGEST FLIGHT SIMULATOR FILES SPECIALIST DOWNLOAD SITE!!!"
