Please define what you mean by "legal" , because i have browsed the download section here, and am not sure what you are getting at ?
Its a term thats been used for Sim Racing mods for years. Some people call these filles "leeched". Basicly how it works is say you have a track you've made in rFactor if I then take your track, add my own textures and slightly modify one corner and then re-release it that file is then know as non legal. In situations like this there are no real legal issues with using that track its just that alot of people like to show respect to the original author and thus boycott these leeched files. As Ben says though, there are also sometimes where the situation is inflicting copyright (this comes when converting tracks between games).