When I find something like this and decide to travel on a hidden-city routing or use one-way of a return ticket, both risks and consequences are all mine. If a travel agent is booking this for a customer then he is also taking responsibility for that customer being able to travel without any problems. Or in other words, it's fine for me to take a risk for myself but not to take a risk for somebody else, especially when that somebody may be poorly equipped to handle issues or questions that later arise.