Why is contributing to the market demand for meat not wrong? According to this article by Ven. Dhammavuddho Thero:


  Majjhima Nikaya 55 ..... The Buddha denied this, adding “Jivaka, I
  say that there are three instances in which meat should not be eaten:
  when it is seen, heard, or suspected (that the living being has been
  specifically slaughtered for oneself) … I say that there are three
  instances in which meat may be eaten: when it is not seen, heard, or
  suspected (that the living being has been specifically slaughtered for
  oneself).”


And another quote from the same source:


  Anguttara Nikaya 8.12 ..... This is one of the discourses which clearly
  shows that the Buddha and his monks ate meat. Also, we see that meat
  from an animal that is already dead when it is purchased is allowed to
  be used, but not if the animal is alive.


In summary from the above and various sources:


It is wrong to kill or directly cause the killing of animals
It is wrong to have a livelihood on the business of meat
It is wrong to consume meat that is from an animal that is seen, heard or suspected to have been slaughtered specifically for you
It is ok to purchase and consume meat from the market (already dead before you arrived at the market)


My thought is that although the last case is not wrong due to an individual not being the direct cause of the killing of the specific animal, the individual is still contributing to the general demand that drives the meat market i.e. encouraging the supply of meat by butchers. If there is no demand, then there would be no supply. Unlike a tiger killing its prey, a butcher only slaughters the animal if there would be purchasers who would buy its meat.

So, why is this not considered wrong by Buddhists?

EDIT: Preferably, according to the Theravada tradition.
Because buddhist morality is not about regulations and punishments for bad deeds. It's about the effect actions have on your well being, and others' well being. It's "medicinal morality", rather than "legalist morality". And what is bad for the mind's health is to act with malicious intentions. 

This perspective on morality is completely different from our western morality which is still influenced by a christian view of exterior punishment for bad deeds. The punishment comes from you alone and is thus rooted in your intentions behind the deeds.

In this case, this is just one more instance of how suffering in the world is innevitable. It's not that being vegetarian is irrelevant. It is obviously a wholesome thing to do. But there are far more important things to worry about regarding what is good and what is bad for the mind, because when you buy meat you don't have the intention to kill animals. Just like when you walk the street you have no intention to kill bugs, even though that is completely innevitable in the long run.