Agreed, there is nothing inherently wrong with cast or swaged lead bullets. In this case, Glock is specifically warning against using cast lead bullets in Glock's polygonal-rifled barrel. For some reason, a traditional cut- or broached-rifling barrel is less prone to the kind of lead build-up that seems to be a problem with Glocks.
I have never personally used cast lead bullets in a Glock, so my information is admittedly second-hand. But I do believe that this warning is given at least partly because there's a technical reason for it, and partly as you suggest, just to avoid liability.
Frequently, I rely on advice that I consider authoritative, especially on topics such as this. Here are three excerpts that I think are worth considering.
THE COMPLETE GLOCK REFERENCE GUIDE Third Edition, © copyright 2006 PTOOMA Productions LLC, page 36:
"Lead Ammunition
The problem with lead bullets and Glock factory barrels starts with the rifling. Traditional rifled barrels incorporate grooves and lands. The lands actually engrave the lead bullet and cause it to spin as it moves down the bore. Glock rifling, referred to as polygonal poly(many) gonal(sided) incorporate multi sided flats that are not conducive to use with lead bullets. The lack of lands in the polygonal rifled Glock barrel tend to allow a lead bullet to skip down the bore rather than spin, leaving larger lead deposits while creating build up and reducing bore diameter. A restricted bore builds excessive pressure. Excessive pressure coupled with an unsupported chamber can leat to a Glock KB (Ka-Boom).
Many IDPA and USPSA patrons shoot lead from Glock factory barrels but are meticulous at maintaining a clean bore. They are keenly aware of the dangers they impose upon themselves. The easiest fix to this potential problem is to simply use jacketed or plated bullets in your factory barrel. Use an aftermarket standard rifled barrel for lead bullets.
Federal Ammunition has offered a lead .40 S&W cartridge that works very well in Glock handguns. This is because the lead is very hard and does not foul the barrel at the same rate as traditional lead cartridges."
(The comment about unsupported chambers is old news and is discussed elsewhere on this Forum. The comment about Federal Ammunition cannot be verified, but since the book is over seven years old, I would research that before making any assumptions.)
ARMORER'S MANUAL, © Glock 2009, page 82:
"Use of non-factory loaded ammunition or ammunition loaded with non-jacketed (lead) bullets will void the warranty." (emphasis is mine)
The Glock in Competition, © copyright 2005, Robin Taylor, pages 65-68:
"DO GLOCKS FAIL WHEN FED LEAD BULLETS?
Yes, they do, and here's why: The problem lies with barrel geometry. Glock barrels are hammer-forged polygon-rifled barrels. In cross-section, a Glock bore looks something like a stop sign. There are no "grooves" per se, only a series of flat lands. If you measure a conventional .40 barrel, the diameter across the grooves will be larger than the distance across the corners of a Glock. The diameter across the lands will be similar to the distance across the flats, but the total cross section of a Glock bore will be less than the conventionally rifled bore. Because of this, Glocks actually swage bullets down to size. Nothing in the Glock barrel cuts like rifling; instead, the Glock "mashes" the bullet into a polygon shape. Put another way, instead of rifling cutting grooves into an otherwise intact bullet, the polygon rifling mashes the entire bearing surface until corners form at the edges.
Because of this swaging and "mashing" effect, the dynamic forces on the bullet going down the bore of a Glock are significantly higher. I have performed scanning electron microscope (SEM) evaluations on test bullets before and after they were fired from Glock and conventional barrels, and the results confirm that grain boundary separation and tearing occurs to a much larger degree.
The design of the Glock barrel was intended to optimize performance of jacketed bullets with inconsistencies. As such, with the crappiest jacketed bullets you can find, Glocks will usually outperform most other pistols. However, with lead bullets, lead gets stripped off the bullet as it travels down the Glock bore. Copper holds together better because of its ductility, but the lead tears away from the slug and is left in the bore as deposits. The first few lead slugs leave flakes of lead behind. With heat and pressure, these flakes eventually "solder" to the bore. Even with the unusually hard and small-grained Oregon Trail bullets, this occurs some. Combine these progressively higher pressure levels with the partially unsupported chamber in a Glock, and you have a potential problem."
The author goes on to describe some testing he did in connection with a lawsuit, where he was able to induce a failure in a Glock 22 by using lead bullets. I encourage you to buy or borrow that book to read the entire article, I'm simply tired of transcribing it.
That has been sufficient for me to avoid using lead bullets in Glock barrels.
Chris