it depends. Climbing volcanoes isn't always risky - I've been to the top of Vesuvius and walked on other ones (including one in NZ whose name I forget, but it wasn't this one).
However, if the warning level had been raised to 4 out of 5, it beats me why a ship would be running excursions there at all. The passengers may well not have known about the risks, they just thought they were going to see a tourist attraction. But those in charge should surely have known of the warning and acted on it.