Actually, that one might be a myth ; ) The reason for the saltpeter was supposedly to make them burn smoother or something. But they do add things, for flavor and to make it burn well and not go stale before it gets sold.
Before viagra, much of the business that "fertility clinics" did had to do with male impotence problems. There were studies done, and IIRC, upwards of 70% of the men were heavy cigarette smokers, and Marlboros were disproportionately represented among the smokers as a preferred brand.
There are other things that contribute. Carbon monoxide, which I think has a long half-life in your body and displaces some of what should be oxygen in your blood, and the vasoconstricive action of nicotine are both things that wouldn't help, and they're common to all brands.
But the Marlboro-impotence thing was I think the inspiration for the controversial California anti-tobacco ad campaign in which the Marlboro Man's cigarette droops.
I better not quit smoking then or I will have to live my life in pain and embarrassment.
(not that it's not already painful and embarrassing to be me) ;^)