I think it is the traveller's fault. It is a person's own responsibility these days to personally make sure about visa requirements/security notices/carry-on restrictions/etc before getting on a plane.

I learned this the hard way: I transited through Shanghai on my way from SE Asia to North America once. I repeatedly asked the travel agent if I required a Chinese visa, even though I was only switching planes at the airport. He said no every time. I arrived in Shanghai and was promptly taken into custody by the local authorities for arriving in China without a visa. They took my luggage, my passport, and tickets and placed me alone in a holding cell. Fortunately, about 45 minutes later, the police supervisor came by and let me go, having verified my identity and my onward travel plans for that same day. I was sent on my merry way with a reminder to get a transit visa the next time.

The lesson is: it is your own responsibility to verify what documents you need, not your travel agent's.