The concern is that given what we've learned about microorganisms in space, this may not be enough; "Extensively sterilized" is never "Perfectly sterilized", at least, not without building it out of solid metal and annealing it in space. There was also a last-minute variance granted to the drill bit on Curiosity, which was contaminated between sterilization and launch by briefly opening the box in a lower-spec clean room.
http://www.space.com/13783-nasa-msl-curiosity-mars-rover-pla...
As we enter an era of inexpensive launch capacity, the sterilization step is actually so expensive it may exceed the cost of the launch vehicle. There is a case to be made for dropping these protections: They're a barrier in the way of a widespread robotic exploration mission, and a manned mission is going to be implicitly contaminative, since you can't run humans through an autoclave. The most conservative route to detecting life before sending humans is years and years of robotic launches to a variety of regions, with several sample returns. It's arguable, but I'm not sure we have the stomach to do that before the 2030's, there's no sign of the major expansion of NASA funding that would be required for the decade of exponentially escalating robotic precursors to a human mission, on the horizon. Given that we're apparently not going to protect Mars from life, that brings into question why we should bother pretending that's what we're doing today. There have already been substantial transfers on previous landers and impactors.