The media often does these things essentialy demanding legislation.
If you tell people something enough from enough sources they start to feel it is a major concern even if what you tell them is not true.
The Iraq war was rather interesting in that respect. Enough people were sure of the "weapons of mass destruction" because multiple sources gave proof. Years later it turned out the source for most of it was a single guy gaining US protection and citizenship saying most it it. The white house was repeating it and the press was repeating it and it appeared that information was coming from multiple sources.
It appeared the information was being corroborated from multiple sources, but in reality was all from a single original source.
People are naive. If multiple news agency, ATF talking heads, and politicians like Hillary Clinton, Joe Biden, and Eric Holder are saying the same thing in media the people are given the impression something must be done. They are then not surprised when legislation is passed reducing thier rights to fix the problem.
The same thing was done before the first assault weapon ban. Lots of media attention on those type of weapons even though they were rarely used by the criminals they were portrayed with. While most of the gang members of the early 90s were using handguns for most things the media managed to create a panic about assault weapons, even though long arms of any type accounted for a small minority of crimes or murders.
Manipulating the herd.
Yes there is without a doubt guns in Mexico from the US. There is only a few thousand firearm permits required by law to own a gun in the nation of over 100 million people. Laws limit people to firearms that will not protect them from both the cartels, criminals or thier own corrupt government.
A significant portion of the cartel fights involve individuals in body armor and uniforms. Often LEO or military uniforms, but sometimes even thier own uniforms with the initials of thier own cartel. They look like professional uniforms worn by some paramilitary team.
So yes there is citizens who fear for thier safety and want to protect themselves with firearms in Mexico and will obtain guns for that purpose. So thier is an illegal market for such guns.
The cartels however have so many sources and contacts for weapons they are not limited. They can aquire guns from South America, the Mexican government, and even overseas.
People making hundreds of million dollar submarines in the middle of the rainforest that can travel thousands of miles are not going to have any trouble aquiring small arms.
They can purchase them and they could even make a batch of tens of thousands for less than the cost of making some of thier submarines.
They are working with billions of dollars.
The corrupt government of Mexico is very oppressive. They set up checkpoints and roadblocks with the military. They travel extremely well armed. They do not want thier citizens with arms that can be used to resist thier oppression. Many in the government and military also have ties to the cartels.
The infamous Los Zetas are in fact founded by those trained by the US to fight cartels who simply switched sides. The money is better on the cartel side.
They have set up camps identical to those the US trained them with (which have been located several times) and train new recruits.
So while restrictions on weapons will not limit the cartels ability to aquire what they need, they could accomplish goals of both the US and Mexican governments. Limiting thier civilian populations ability to aquire and own weapons capable of posing effective resistance.
The US wants to remove the ability of citizens to obtain weapons not registered, especialy capable weapons that can be sold FTF. The Mexican government several steps ahead wants to remove the ability of thier citizens to aquire the weapons at all. That way only the cartels and the corrupt government will be well armed.