Nearly all guards, almost universally, ignore Penalty Fares schemes. I am not sure what else anybody expects them to do...
Getting somebody to buy any ticket is often impossible, let alone charging a Penalty Fare. I made a journey from Stafford to Crewe on WMT yesterday, and when the guard came through he gripped my ticket, and then asked the next passenger for theirs. They presented, and this is no word of a lie, a receipt from Tesco. The guard just said right, and have you got a ticket? No. Have you got any money? No. Right. You'd better get off at Crewe then hadn't you. Yes, that's where I'm going anyway. The guard simply walked off in exasperation. I have no idea what else they might be expected to do. This gentleman clearly knows there's very little chance of anything coming on him for not buying a ticket. So much so that he wasn't a bit embarrassed about presenting an effing Tesco receipt.