The metaphor of "war" attempts to create the perception that the drug issue is a "threat" coming from outside the boundaries of the United States. Therefore, it must — and can be — contained on "the other side," if not eradicated at the "source" (see A Disastrous Metaphor: Waging Domesitc War). Thus, if any aspect of the "threat" of drugs, crosses the border, it is "spilling over."
Goods sold by the Mexican cartels are excluded from legitimate international trade relations; hence, they are denied direct access to standard banking channels for currency exchange and transfer of large sums across national borders. So the cartels become inventive — using U.S. perfume merchants and Mexican currency exchange businesses. The money is not "spilling over." It is flowing between markets.
The general conclusion of a number of writers is that the cartels use U.S.-based gangs for distribution precisely because they don't want to get entangled with U.S. police and courts. For the same reason, they don't want to increase acts of violence north of the border. In this sense, the existence of a jurisdictional border actually prevents "spillover."