You're right of course. No ammo manufacturing company wants to be the one to get sued when some dumbass loads hot factory loads in his great grandfather's Trapdoor Springfield.
With that in mind, a few years back I had a hankerin' for a lever rifle in 45-70. After reading about the Marlin's of the time and not being incredibly impressed with the reports of poor production quality (Around the same time I had also bought a new Marlin XLR in .35 Remington and it was a total lemon) I decided to go with an Rossi M92 in .454 Casull. Hot Casull loads, like Speer Gold-dots, came out to around the same muzzle velocity as factory 45-70's. They tended to be (slightly) cheaper too. Another plus was that the Rossi weighs around 6.5 pounds loaded with 10 in the magazine and 1 in the chamber. I'll probably still end up with a 45-70 or 2 one day, especially when I start reloading. From what I'm told you can actually get the cartridge up to it's full potential that way. For now though the .454 has been a good choice for short-medium work.