Never seen a lead only range but the only reason that would make sense ( IMO ) would be to prevent damage to the backstop or perhaps the backstop is not that thick to be able to withstand heavy FMJ loads.
A lot of it is due to backstop. A range I belonged to decided to allow fmj bullets, and the bullets were passing the shooters on the line going the other direction. Some of the older ranges were designed around bullseye shooting with primarily target 22's and 38 wadcutters and the plating couldn't handle the hotter loads.
I wouldn't join a club that wants you shoot lead only. Most gun stores that sell factory ammo, sell FMJ ammo 90% of the time. Besides lead is a health hazard & a pain in the @ss to clean in your guns.
To msg.drew
I guess you enjoy sniffing all those lead particals in the gun range.
Sadly to say iv'e lost 2 friends from lead posioning who were avid shooters in my clubs. I would only reload & shoot lead as a last resort in my world.
Some of the indoor ranges iv'e been to last year had 1/2 @ss air systems.
Never see me their again for the life of me. I guessing your range is an execption. As for testing lead in my blood I do that every year. It's good advice, all shooters should get tested for lead once a year.