I fail to see how loading a single round could cause an accidental discharge. If you load a single round into the chamber through the ejection port it can damage the extractor. The extractor is only designed to engage the rim of the cartridge by the round sliding up into the extractor as it enters the chamber from the magazine. To load a single round, do it from a magazine. Because of the extractor design, yes it is common to all 1911's.
Even if you were to load a round through the ejection port, it would be extremely unlikely that a discharge would occur. The firing pin spring is strong enough to keep from firing the pistol when the slide slams home. If there is a magazine in the pistol and you load a single round through the ejection port, the slide will stay locked back because the magazine is empty. Even so, the hammer won't fall unless you squeeze the grip safety and then pull the trigger.
A lot of 1911's are advertised as 6+1, 7+1, 10+1, 12+1, etc. This refers to having a full magazine and an additional round in the chamber. Most people carry a 1911 in condition 1 (round in chamber, hammer cocked, safety on), also called cocked-and-locked. Carrying a 1911 in any other state is generally considered to be unsafe. A round in the chamber with the hammer down is asking for trouble, as you would have to squeeze the trigger and let the hammer down slowly. If you try to do this with one hand and drop the hammer, the slide WILL seriously injure your thumb when the pistol fires. If you drop a 1911 with a round in the chamber and the hammer down, it could cause a discharge if the hammer strikes the ground.
This warning sounds like a typical corporate CYA notice. They must assume people are so stupid that they will forget there is a round in the chamber after they just put it there. The only thing I can think of would be someone might load a single round through the ejection port without a magazine in the pistol. Since there would be no magazine, some idiot might think the pistol was unloaded and squeeze the trigger. This is called natural selection. It's how nature removes the morons from the gene pool.
Since we all know that a firearm is ALWAYS treated like it's loaded, there is the same chance of the pistol going off accidentally with a single round in the chamber as there is with a full magazine and one in the pipe. The 1911 design is over 90 years old and really hasn't changed much. If single round accidental discharges were a problem, we would have known about it DECADES ago.