People climb mountains, bungee jump and sky-dive because death, while a possibility, is not a definite outcome of these activities. His Mars trip, on the other hand, is essentially a suicide mission.
Davies clearly stated:
Most people react with instinctive horror at the suggestion. I recall my own sense of discomfort when I met an aging American scientist who claimed to have trained for a one-way mission to the Moon in the pre-Apollo days. And in the case of the barren Moon, that reaction is largely justified. There is little on the Moon to sustain human life. Mars, however, is a different story. Because of the planet's relatively benign environment, it is theoretically able to support a permanent human presence. If provided with the right equipment, astronauts would have a chance of living there for years. A one-way trip to Mars need not mean a quick demise.
Perhaps the knitting poet from San Francisco did not understand such subtlety. If going to Mars (following Davies' suggestions, as opposed to Soviet cosmonaut style - if the slightest thing goes wrong, you'll be airbrushed out of all our photos) is suicide (since death is certain), then we are all suicides, since we can all expect to die.