This from Your Guide to Oyster Daily Price Capping {pdf}
Once you have reached a cap, you must continue to touch your Oyster card on the card reader on every trip. If you do not do so, you may be liable to pay a Penalty Fare or you may be prosecuted.
In other words: "Even if your travel is fully paid for, we still want to know where you are."
Is it just me, or is the Oyster logo half a pair of handcuffs?
