Lets say Curiosity crashed into mars... that would be a HUGE problem for NASA (3 billion dollars wasted, people would say). But if a couple of astronauts died on the way to Mars... I think that would only strengthen the resolve of the American public. No politician would go on record saying their life was wasted. Instead, their death would be framed as heroic (rightfully so), and I think there would be a push to try again, and to try harder.
Given that logic, what NASA views as risk mitigation (send a robot to mars instead of a person, so we can get better at it before risking lives), is really is a very risky strategy from their organization's perspective.
There is probably data out there that backs/disproves my general logic here (sentiment of nasa after astronaut's deaths, vs after loss of robots). I'd love to see it.