Stan, indeed, the question gets asked with each and every spacecraft mission we fly. NASA has a "Planetary Protection Office" for exactly this purpose, and it is intended both to prevent undue contamination of worlds we visit, as well as to protect Earth from potential "Andromeda Strain" problems when samples of other Solar System objects are returned for study. All missions that will visit any object that could remotely harbor life must clear a very steep bunch of hurdles before flight, such as extreme cleaning of the spacecraft if it is ever going to come into contact with anything (as opposed, say, to a flyby situation as with New Horizons at Pluto).
In the case of Cassini, first off the spacecraft will be basically vaporized by the time it gets only a small percentage of the way into Saturn's atmosphere, reduced to its constituent molecules by the intense conditions of the saturnian atmosphere. And those molecules are all of materials that are already present in other forms on/in Saturn, with the exception of the nuclear reactor's fuel. But that is a few kilograms of material, and compared to the mass of Saturn it makes a drop in the ocean look like an ocean of its own, it's so insignificant. Given that there is absolutely no evidence to infer that the planet itself has any kind of consciousness (I do not believe in such things, nor does any physical scientist I know), this addition to the planet's inventory of stuff is truly negligible, in the literal sense of that word.
Now, the satellites that orbit Saturn are another matter, and a couple of them could well be habitable, at least by primitive life forms. And as the text I pasted in demonstrates, incredible effort is being expended to ensure that Cassini cannot influence any of those potentially habitable worlds during the grand finale. Truly heroic efforts being made in that respect. So I would argue that the mission team has performed far and above any ethical threshhold in designing and implementing this planned trajectory.
I believe that NASA and other space agencies make every effort to minimize the impact of our exploration hardware on the places we go to explore. I believe it is a very acceptable price to pay in order to do that exploration. I believe humanity MUST explore, it is hardwired into our nature, and that if we stop exploring, we'll atrophy as a species. It sounds overly dramatic, but I really believe our survival as a species depends on us continuing to explore, always push to the next horizon. (Cue cheesy Captain Kirk speech )