You turned around too soon, as just up ahead was the entrance to Loop Creek Canyon, so named for the Milwaukee Road tracks that descended the North side, looped around and then down the South side to near Pearson, ID.
The Hiawatha Trail, as it is called, is managed & maintained by the Lookout Pass Ski Area on Interstate 90 right at the Idaho / Montana state line. In exchange for their hard work & efforts, the USFS allows them to collect a day use fee for any & all who use the trail. The " USFS " persons you encountered may have actually been volunteers from the Ski Area.
This really is a great way to see a beautiful part of the Idaho Bitterroots, and a historic portion of Milwaukee Road R of W. You can rent bikes, helmets, and lights from the Ski Area,( they'll even loan you a bike rack if you don't have one. ) then drive down I - 90 to the Taft, MT . exit and up the USFS road to either East Portal,MT. or Roland, ID. Here's the best part : After riding downhill for 12 miles around Loop Creek Canyon, through 6 tunnels and across 7 tall steel viaducts, the Ski Area has a Bus ( extra fare ) to bring you back up the hill to your car!!! HOWEVER be aware that the bus only carries passengers as far as the Roland trailhead. This means, if you park @ East Portal, a two way trip through St. Paul Pass tunnel, all 8700+ feet of it. Of course you do have the unusual pleasure of riding under a state line, clearly marked on the tunnel wall !!! Give it a try next summer.