[careersuicide]

In general great effort is made to ensure that probes don't contain any biological contaminates so if we do discover signs of life we're very certain it wasn't a microbe that hitched a ride.

As far as contamination from drilling (due to lubricants or something else entirely) I imagine that the desire for general cleanliness is for much the same reason. If we're going to spend hundreds of millions of billions of dollars on a probe to another world we want to be really confident anything interesting we find is actually from that world and not Earth.