Two things hold the cam in place - the pulley keeps it from moving rearward, and the angle that the lifters meet the lobes, which are tapered, provides enough rearward force to keep it from riding forward. This also rotates the lifter, creating an even wear pattern. If you switch to a roller cam with no bevel in the lobes you will need to use a cam button.
Mike, I've always known the lifters rotated and was designed into both the cam and the lifters. That is great info as it explains just how the design works. I learned something for sure. Thanks for posting that link.