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.
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.