We still understand how things work in the body very generally or not at all. They made their best guess. It makes sense that putting air in the lungs would be a good thing.
Then, their guess gets implemented and it works. So people use mouth-to-mouth for years. Then along comes AIDS and it takes years to notice that folks are skipping the mouth part and then more years to accumulate enough data to actually measure and make a determination whether one method or the other works best--you can't really do a controlled experiment. "OK, all those who volunteer to have a heart attack and get Push Only treatment, please move to the right side of the gym . . . " :)
So give 'em a break. They saved a lot of lives even though the technique they taught was overinclusive and kind of gross.
Medicine is like any other field of learning. New knowledge is constantly being aquired, but we don't know what we don't know. I've must have learned six variations of CPR since 1969.