Why is contributing to the market demand for meat not wrong?
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.