It's normal, they sell weak springs though that go in the back to help prevent it, but I've never used 'em.
Yeah, it can slam fire, but usually if there is a problem with the pin, OR if you drop the rifle directly on the muzzle from an attic hatch 10 ft. onto solid concrete. I know because my partner in SDM school did just that. If you leave the safety off it'll do the exact same thing but release the hammer, seen that too. It only discharges one round in either case, as the kinetic energy sufficient to make it do that is far beyond what happens on a normal basis.
Some rifles print on the primers worse than others for various reasons, usually due to heavy springs/buffers. Never seen one go off under normal circumstances and I've used/seen a LOT of AR's/M4's.
It's worth noting that the military uses hard primers. The CCI military primers are also the same mixture as their BR primers, just with a thicker cup. It can more easily discharge under normal circumstances with the BR primers.