It is wise to be very careful about not contaminating other planets with microbes from Earth. That is why NASA decided to let the Galileo probe burn up in the atmosphere of Jupiter so as to avoid harming what could be an ecosystem on Europa. As far as the moon goes we have been there and studied it enough to know it is a lifeless hunk of rock. The impact caused by LCROSS was nothing compared to what nature throws at the moon all the time in the form of other rocks.

Also this is not the first human made object to hit the moon:
www.space.com/scienceastr...crashes.html