In reply to:
Bank charges can be levied because they are specified in a contract that you have previously signed.
That is precisely the same, the bank specify in a contract what services are available to you and the limits on those services. Where in your contract with BT does it say that you can use the facility in question? If it doesn't say that you can use it, you may not do so. Using facilities to which you not entitled are likely to require not just payment but penalty payments, such as those for an unauthorised overdraft.
In reply to:
Anyway, clearly the way to proceed is for BT is to introduce a password-type system as they have already done. I'm not clear on why the menu options still allow non-password access too though
Probably for much the same reason as sweet shops do not have locked doors over all their counters to make it clear that unauthorised taking of their goods is not permitted. The cost would be ridiculous considering that the majority of the population have an idea of what they can and must not do. The system you suggest would be paid, in the main, for those who do not need it. Only those who break any good practices they can get away with make it necessary. This way is far fairer at least for the well behaved - but the habitually anitsocial may not see that.