Is the Mars rover program a scam?

I've always been under the impression that the Mars rovers were sent up largely with the intent of seeking to discover if there is or has ever been life on Mars.

Turns out, they're not going to send this 2.5 billion-dollar gizmo that was sent to explore Mars anywhere near where there could conceivably actually be life, i.e. where there's water. Supposedly because of concern over the potential for introducing Earth microbes into the Mars ecosystem.

Say what? Apparently 2.5 billion doesn't buy an explorer that's clean enough?

So under what circumstances would they ever be utterly certain that anything they send up is completely, unquestionably sterile?

So is this rover program another expensive bill of goods just like the shuttle was?