Reports such as this one claiming conditions are impossible for life on Mars never seem to have biologists among their listed authors.
A biologist could have told them Earth organisms have enzymes specifically designed to counteract oxidants such as H2O2:

What the Heck Do you Mean - Oxygen is Harmful??
"The superoxide anion mentioned above (O2-), if available, can be
converted by an enzyme called superoxide dismutase to hydrogen
peroxide (H2O2) and molecular oxygen (O2) by the following reaction:
2O2- + 2H+ => O2 + H2O2.
Obligate aerobes - organisms which absolutely require O2 (many, many
microorganisms, including bacteria, and a bunch of macroorganisms -
like us) have this enzyme. As it turns out, H2O2 is also toxic -
bummer! - get rid of one thing and another one pops up! But - cells
are "smart." While H2O2 is dangerous, this stuff is significantly less
toxic than superoxide anions. But, cells may also have yet another
enzyme (our cells do - as do obligate aerobes) called catalase. This
enzyme converts H2O2 to H20 and O2 in the following reaction: 2H2O2 =>
2H2O + O2."
"So, all strict aerobes have _both_ of these enzymes.. and although
some bacteria do not have catalase, most _will_ have superoxide
dismutase. If they don't have superoxide dismutase, then they'll have
an enzyme which uses some metal ion (like manganese or some such
thing) which destroys the superoxide anions. Therefore, O2 is
necessary for all aerobes, but certain forms of O2 are dangerous as
well. Consequently, there are very sophisticated systems which protect
cells from the potentially toxic effects of this critical life-giving
substance."
http://people.ku.edu/~jbrown/oxygen.html

Catalase.
"Catalase is a common enzyme found in living organisms. Its functions include catalyzing the decomposition of hydrogen peroxide to water and oxygen. Catalase has one of the highest turnover rates for all enzymes; one molecule of catalase can convert 5 million molecules of hydrogen peroxide to water and oxygen each minute."
http://en.wikipedia.org/wiki/Catalase

Interestingly, both of these sites mention bacteria convert
oxidants such as superoxides and hydrogen peroxide into water and O2.
Enzymes to deactivate superoxides also occur in anaerobic oxganisms
which might be the type more likely to currently appear on Mars:

Anaerobic Microbes: Oxygen Detoxification Without Superoxide
Dismutase.
Science, Vol. 286, No. 5438, 8 Oct 1999, pp. 306-309.
http://www.sciencemag.org/cgi/content/full/286/5438/306

So the presence of oxidants in addition to the observation of oxygen
produced in the biology experiments might in fact support a biology
explanation of the Viking biology results!!
If in fact life evolved on Mars in an earlier less hostile time in it's history then we would expect them in fact to have evolved defense mechanisms against such
oxidants as has happened for life on Earth.

Bob Clark