This statement is sensationalist and not true, as the maximum load on a Oyster Card is £90.
London transport must send thousands of payment requests to banks each day. Banks will honor them without checking anything, as there’s no way (apart from contacting the customer) for them to verify that London transport has the right to request those transfers of money to them (“here’s the form this customer signed last year” doesn’t say anything about whether the customer withdrew permission later)
The contract between the bank and London Transport likely says something about the amount of money they can request to be transferred to them each week, month or year, _may_ limit the amount per item, and says the contract will end if London Transport misuses the trust the banks give them, but that will be about it.