That's not the full description of what happened:
It's also not correct to say, "we don't do prams" - it's that, "we don't do prams for passengers who aren't disabled, elderly, or less mobile". They could've taken 10 seconds to explain that passenger assist is for those groups, not just any passengers who have prams or heavy luggage. Given the Passenger Assist forms I've just sampled, most of them don't actually require selecting a disability, mobility need, etc; you can indeed tick that you need help with luggage and proceed to booking. It doesn't sound like RPM's pal intentionally booked it knowing it wasn't designed for them.