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?
So if one is elderly, or forgetful, or even youthful and forgetful, the Oyster bearer is liable to be clobbered if the required ‘touch’ is not carried out! This is another of the ruses from Gauleiter Ken, whose sole desire is to relieve Londoners of all their earnings, and so bring the delights of the communist state to our once green and pleasant land!