The hell we can't.
Legally, you can't. If something is not obscene, it is legal to produce. Therefore, paying actors to perform in the production is also legal. The reason prosecutors don't try this move is because they know they'd be laughed out of court.
Think of it this way- the 2nd Amendment recognizes the right to bear arms. Could the government make an end-run around this right by outlawing the production of firearms?
Whether or not they film their crimes is irrelevant.