First of all, Bubblyrat, of course Samaritans are allowed to say "commit suicide" - where on earth did you hear they couldn't? That's the question Samaritan volunteers have to ask in each and every conversation. It's part of the training and part of the whole ethos of what happens.
And secondly, of course the volunteer will be doing everything they can to find out where the suicidal person is, in order to help them, but if a caller doesn't give an address (specifically chooses not to, in some cases) it's very hard to see (a) that the caller wanted to be prevented from suicide and (b) how to help them. A phone number does not generate an address, even if you could find the number they called from. If the Samaritan DOES know the address or phone number then they will do everything possible to help the caller - I've been in that position and I know. I once had half a phone number before the caller rang off. There are no words to describe how I felt about that.
I'd suggest if you're agonising about how and why Samaritans operate you should simply call and ask them - it's not a secret society! But there is a prime rule of confidentiality (hence not using any method to trace the call), and a prime rule of allowing callers to make their own choices, which might mean allowing someone to kill themselves if that's what they really want. With an actively suicidal caller, any volunteer will keep asking for an address or some other way to trace the caller, so your mythical person who goes over the edge and expects to be rescued will have been asked and asked again for contact details.
Please don't go spreading unfounded myths...and please also remember that the Samaritans is an entirely volunteer-run charity. If you think you can do better, or as well as they do, by all means put yourself forward to your nearest branch and discover the reality of what happens and the kind of call they receive.

Anne