The point is that, like a lot of places, the Les Arcs trail map is a bit rubbish. It's definitely not all fire roads (Black 8 is a well-worn classic obviously, and there's quite a few other decent bits and pieces on it) but it does contain quite a few and it most definitely doesn't give much info on the great singletrack riding to be found at Les Arcs.
If you REALLY just want to turn up, pick up a trail map at the lift pass office and use that for a day's riding, you'd arguably be better off at Sainte Foy, La Thuile or even Tignes if you're in the Tarentaise area.
On the other hand, with a proper map and a bit of imagination, it's not exactly difficult to find great trails in Les Arcs. Might involve a few adventures along the way, but loads of stuff to be found.