Are US gun shops and drug users responsible for Mexico's violent drug cartels?

A provocative Monitor opinion essay makes the case that America is morally responsible for the violence committed by Mexico's drug cartels:

Mexican drug lords exist to feed the US drug market. And they get their guns through the US weapons market. We give the bad guys their money by buying their drugs; we sell them the guns that enable their continued existence; and they threaten a nation of more than 100 million people at our border.

What's the solution? According to writer Jacob Bronsther, it's two-fold:

As to the guns, there aren’t any magic policy bullets, but closing down the gun shops arrayed at the border, possibly though zoning laws, is a good start. The 2nd Amendment doesn’t cover Mexican drug lords, gladly. We should also increase the penalties for smuggling weapons.

As to the money, we need to think harder and faster about stemming the demand for drugs, and find the maturity to discuss the legalization and regulation of some recreational drugs, which would funnel the funds away from the cartels, even if we ultimately reject such arguments. A less dramatic start, however, would be a public advertising campaign that informs people where the money goes when they buy cocaine.

Do you agree that America is morally responsible for Mexico's drug violence? If so, what obligations follow? Share your thoughts below.