This is an argument produced by Manuel Cortes of the TSSA union, acording to today's Metro free newspaper (London). It is true as things stand, but it is entirely possible to produce a ticket machine that can dispense the full range of tickets and corresponding prices. A major step towards this would be simplifying our ridiculous fares structure. One obvious thing would be not to have a fares system where a ticket from A to B and another concurrent ticket from B to C works out cheaper than a ticket from A to C on the exact same trains.
Cortes makes the point as if it is technically impossible for machines to be as good as humans at selling tickets. It's his job to keep people in work. In a sane world, we wouldn't try to create mundane jobs for humans to do. Everyone would do socially constructive work that a machine can't (yet) do, or no work at all. Machines would be perfectd so that they are as good as or better than humans.
The idea of buying a ticket off a person standing behind a bit of glass is so antiquated with today's technology as to be ridiculous. Setting aside the obvious development of smart ticketing, itself argued against on spurious grounds but because it takes jobs, the logical thing for expert ticket sellers to do would be to guide people using the ticket machines, or buying tickets on the www.