They brought it with them (in the form of food and water and what was already in their body), taking it back should have been in their calculations.
By bending these "rules", we have now contaminated a once pristine environment, and new scientific discoveries will have to eliminate our contamination on anything they find. From bacteria to tardigrades, to liquids and other potential chemical contaminant.
... really? This feels like nothing but unrealistic idealism to me. Wherever we've gone exploring as a species throughout our history, we've left our mark (usually bad). You might dream of a reality where we change nothing at all, but it's just not how things work out in reality. And honestly, leaving a bunch of waste, which is most probably entirely sterilized and cleaner than your kitchen by now, on an uninhabited astral body with no ecosystem or atmosphere, seems a bit less egregious than wiping out local wildlife or cultures.