Specklebelly,
I think rust IS the issue. If you leave one of these guns sit unprotected for 6 months, I'd bet you would have rust, even indoors. The conditions these guns are shipped, or stored in at distributors, and gun stores, may also lead to quick rusting, so to protect the guns, and their investment, they grease them up. Cosmoline is still readily found in the military surplus guns, it does do it's job. Some surplus guns come out of 60 years of storage, and no rust. They're a pain to clean off, but worth the time invested to have a rust free gun.
Don