They are only sharp when you buy them. Most all the disposable saws are made of impulse hardened teeth, and they can't be sharpened with a file, because the steel is harder than the file. You have no choice but to throw them away.
It was a dying art until around '96 when a couple guys started a making quality saws in America again, the company was called Independence Toolworks. Lie-Niesen bought them and making saw, but others have come to surface also, and there is a resurgence again, case in point the handsaw I milled out of bronze above. Milling a slotted back is what Pete Taran created, in '96.
There is no excuse for not using a sharp tool. Hand sharpening was something that every craftsman knew how to do in days of lore, but that is not the case anymore, some would rather throw the saw/chain away and get another one. Some people send their saws out to be sharpened, shame on those folks also...this is a basic skill that most craftsmen should know how to do.
Sharpening chain seems similar, it only takes a few minutes to take a file and sharpen the teeth. While it seems awkward at first, it becomes more natural in a very short time. Ok, off my soapbox...:angrysoapbox:
Regards,
TT