Those grams of martian soil will be extremely valuable, literally worth millions per gram. One of the biggest things they're going to look for is signs of life. It makes good sense to take every precaution to prevent contaimination in either direction. We don't want the martian soil to contaiminate ours and we don't want to contaiminate the sample.
Personally, I think the chances of anything dangerous existing in Martian soil are very remote. However, very remote doesn't equal zero. Zubrin's line "Humans don't catch Dutch Elm disease" is pretty shallow. Humans may not catch Dutch Elm disease but Dutch Elm trees do. We're not the only organisms on Earth. Speaking hypothetically, suppose the martian soil was absolutely harmless to humans but somehow proved dangerous to rice, wheat, or corn. Do you think that would have an effect on our food supply?
Space/Science » in reply to I agree with Zubrin.
Look at it this way