The railcard photo-card number is input onto the Oyster as well and a it must be registered at the same address as the railcard. A quick phone call by a revenue inspector can determine the latter.
The railcard photo-card number may be input onto the Oyster but the whole process takes less than half a minute in the hands of a clerk who knows what they are doing. There is absolutely no check on the details of the railcard at all. The requirement for the Oyster to be registered is for TfL's benefit and the passenger's security rather than synchronising with a railcard.
As to RPIs being told the addresses registered to Oyster cards; I think the information commissioner might have something to say about data protection breaches were that to happen.
Quite simply, the actual railcard is required to validate the discount entitlement on the Oyster. If you haven't got it with you then you are obtaining discounted travel without authority in the same way as using a discounted ticket without the railcard is not allowed.
The only aspect of the rules on the TfL site quoted above that I might question is the requirement to always have the railcard on a bus/tram. If the Oyster system shows that only buses/trams have been used that day then there is no revenue loss. It would be an interesting case were TfL to try and take someone to court for that. My guess is that many bus journeys will be using capped Oyster cards where the railcard is needed to validate the reduced cap, so it is easier to say always carry them together rather than explain the specific circumstances where it is necessary. It's similar to the requirement to always touch in on a tram when using an Oyster with travelcard to Wimbledon. The reason is to stop a maximumm fare being deducted by the gateline at Wimbledon which would happen if the travelcard didn't include zone 3. If it does include zone 3 then there is no issue.
