Its the Americans covering their asres from other Americans whingers: If they blindly state what bike it came off, then they can always plead ignorance.
Making a sensible decision about "its clearly from a 1990s bike, probably a 1994ish YZ125/250" invites whinging along the lines of "this item is clearly from a 1993 model, as it is 1.2mm wider, and I want my money back and...".
Same as the way its very rare for them to sell complete motors (or complete bottom ends) - part of it is trying to maximise the money they make, but its also because they don't want to risk having someone whinge about a worn clutch basket or whatever. Instead, they strip it right down and then list individual parts - and are lucky to sell three of the items...
