Should you be able to Delay Repay a Penalty Fare?

[growse]

I've not thought about this at all, so it might be a hilariously misguided.

I've always assumed that penalty fares aren't in any way eligible for delay repay. But my speculation is "why not?"

The wmr website (to pick one at random) says

So, you can claim compensation if you arrive at your destination station at least 15 minutes late and you have a valid ticket for your journey.

Is a PF, once issued, not a "valid ticket"? Front national rail:

A Penalty Fare is an exceptional fare charged if you do not comply with the normal ticket purchasing rules without good reason. It is not a fine.

If I'm issued a PF from, say, Shrewsbury to New Street, and the train arrives 90 mins late, why should I not be able to claim it back (much as if I had a non PF ticket)?