Based on the details in this EP Times article, I tend to think the analysis of the El Paso police is correct.  I see no evidence that these billboards were pointed toward Juarez in any more specific manner than they are on the I-10 (running east west) and thus are visible from both sides of the border.

Not that I’m an investigator, but based on the initial reports, it certainly seemed that the actions and the messages resonated with the Occupy movement. I believe it would have been difficult to create such installations and not leave fingerprints that a US police investigation would find…

There have been hundreds of police murdered in Juarez in recent years and such actions were never advertised in any way in El Paso… -Molly

Las Vegas Man Arrested In Connection With Threatening Billboard Graffiti (El Paso Times)