Why does a person, who knows he is innocent, plea bargain for a sentence -
instead of going to trial.
Because he doesn't have the money to fight the case. For a business the
size of Microsoft, it's more often a question of whether settling will
cost less than winning the case.
However "cost" doesn't just mean legal fees. Microsoft has become heavily
dependent on some legal gray areas such as whether they're actually
selling you a license (as they claim and some courts agree with) or a copy
of Windows (as other courts say). It would so totally wreck their business
plans if any of this were settled to their disfavor in a higher court that
they won't risk a judgement unless absolutely everything is going
their way.
It's not just Microsoft that does this. The media industry works this
way, too. If they can't win, they settle out of court and bury the
evidence and testimony by having the records sealed as part of the
settlement. They only stay with the case if they're hands-down winning. So
they're almost always either settling out of court, or winning. The
settlements cost them money but don't establish any legal precedent like
losing would. Eventually they accumulate enough wins to turn the gray area
into a solid string of legal precedents in their favor.