A pre 1988 small block Chevy 350 has no retainer on the camshaft why and what keeps it from moving?

The pre-88 engines have hydraulic lifters. The bottom of the lifter is slightly dished and the cam lobe is slightly domed; hard to see just looking at it. Anyway, as the cam rotates, the lifters actually spin and hold the camshaft in place. That's why you have to use new lifters when using a new cam, so that they wear together. If you don't then the new cam will eventually walk its way against the timing cover.