"If a person doesn't practice the rule of fidelity, then why would he/she bother to follow a rule about condoms?"

To avoid catching and spreading disease, I'd imagine. It doesn't follow that those inclined to infidelity should necessarily have a death-wish or any desire to harm their spouse by infection, or court the risk of their infidelity being discovered.

"Can't they let the non-monogamists figure things out on their own?"

Quite right, as long as the condoms are freely available as a preventative health measure, alongside sufficient and appropriate education.