But it's not the case. The Etsy seller says "Due to its magnetic functionality, the casing does not affect the function of the card in any way and so can be topped up and swiped just like any regular card. Each card comes with its unique ID included for registration as is accepted at every underground cashier desk I have ever been to."
Surprised no one has thought of doing this before and I love how they're scratchproof, as my Oyster card has really been through the wars. Probably worth getting one while they're hot, as it might only be a matter of time, before Transport for London say "down with this sort of thing", as they did with the Oyster card wands!