Another way to sell it would be as a gamble:

"We'll send you life support while you're there, to the best of our ability, but fundamentally you'll be on your own. And if we can develop a rocket to bring you back, and deliver it to you, then you can come home. But we might never manage to do that: you might be stuck there already."

It's not a new idea, of course. During the colonial era of many of the Western empires of yesteryear, this kind of "one-way ticket" was commonplace. When Britain sent prospectors to North America or convicts to Australia, none of them ever expected to come back again. It was a dangerous, one-way trip.

The only difference is that the colonists on Earth rightly believed that the territory at the other end was capable of sustaining life, indefinitely. They took a big risk, but the reward was worth it - the chance at a new life, their own land, and a new home. The humans who go on a one-way trip to Mars will be completely dependent on Earth to support them: there's no way that they can live up there independently. At least: not yet.