It's not 60% of the radiation that would kill astronauts. It's 60% of the number that we've picked as a lifetime dose.
Which by the way we know is dangerous, but it's a figure that we hope will only cause minor problems like cataracts and joint problems - and hopefully not cancers. With a total astronaut population so small, I'd imagine it's impossible to get any data with decent confidence of avoiding statistical problems.
So no, you're actually wrong. This is proper risk assessment, as anyone not totally reckless with other peoples' lives should do.
The nuclear industry doses are ludicrously low, and there's a good argument to say that if we'd increased the tolerances for nuclear safety just a little bit, we could have made it a lot cheaper and thus saved thousands of lives compared with those we lost mining and burning coal.
But in this case, we can't have astronauts if we try to enforce those kinds of doses, we don't have the technology to get that kind of shielding into orbit - well apart from Project Orion, which has its own radiation issues... So we've gone with a best-guess of what will be relatively safe long-term, but still exposes the astronauts to higher risks than we'd like - but they're willing to live with that.
There's a lot of namby-pamby silliness with health-and-safety. But on the other hand there's a lot of cavalier bollocks that means we kill people we don't need to, because we're not willing to take the time to think about minimising risks. Some of them really easy to minimise too. The construction industry being a good example - where numbers of deaths have plummeted. Take the London Olympics, which were the first to set themselves the goal of building all the venues without any workers dying.
Also if you don't measure the risk, you don't know if you can do something. Until we'd done this calculation we didn't even know if it was possible for the crew to survive even a one-way trip to Mars. To do it without checking that would be stupidly reckless.