On the face of it, this seems to be a legitimate concern, especially considering the reports of Arab artifacts that seem to show up in the border-crosser's left-behind trash.
However, money that would go to "fund terrorism networks overseas" is money that wouldn't go to "funding Mexican drug cartels". And from what I've been hearing over the last few years, particularly coming out of Nuevo Laredo, the Mexican drug cartels seem to be none too keen on the concept of money not going to them.
Further, I suspect that they would be smart enough to realize that were the Islamofascists to achieve their goals, it would mean a significant decline in the market for the Mexican drug producers.
I think what is going on is that Islamics in the USA are buying from the Mexican cartels. The Islamics then sell the drugs in the States and that money goes overseas to fund terrorism.