They won't explore the water-flowing sites to: "minimize the chances of life inadvertently crossing the solar system."
So let's just spread life intentionally. It seems like that is one of our goals anyway -- not necessarily for governments (ie. NASA), but certainly for private space flight.
The aim is to settle the question of whether life exists on Mars, or ever has, before clouding the issue with modern contamination. I say modern just in case of various panspermia-type possibilities.
That doesn't preclude eventually spreading life to Mars. Even if living indigenous bacteria, or even plant life(!) were found - I think this is the most ambitious scenario now conceivable, the 'Mars mat' of fiction suviving in caves - it likely wouldn't be an argument to stop colonisation, though it might have bearing on arguments about terraforming.
So let's just spread life intentionally. It seems like that is one of our goals anyway -- not necessarily for governments (ie. NASA), but certainly for private space flight.