Whenever I see a recipe that asks for beans go "look over and pick out any rocks or bad shit" I get confused and worried. I've never found small rocks or encountered a bad bite when I use beans, both bagged or canned. Have I been ingesting rocks and not knowing or have I been extremely lucky and have gotten the shit that has no rocks?

And why do they have rocks anyways? Shouldn't that be processed and sifted out before hand?