It could mean you could be charged twice for your fare, for example if you have a weekly travelcard loaded on your Oyster card and when getting on the bus you are charged on your bankcard.
Now that Matt Blaze has released his report, everyone should be paying attention because my experience is that it will not be very long before technically-savvy activists begin to retaliate, just like they did against major bankcard facilities in reaction to the shutdown of WikiLeaks ability to accept credit cards for payment.