A malicious user could bitcoinczechia set up a swarm of identities to act as nay-sayers and therewith deny some or all others from performing transactions. But such a bank would have to keep its own records – both as a practical necessity and as a legal requirement – and those could be obtained by the authorities. Whereas cash can be laundered tracelessly, through a cash business like a casino or restaurant, which can perfectly innocently be expected to have lots of cash coming in and no way of knowing where it comes from. But the need for every transaction to be publicly verified means that you are tied to all your transactions.