Last time we went to the U.S., in March to West Texas, we crossed back in at Laredo.
The Aduana said they were required to check every vehicle that crossed into Mexico. I think it is only logical that foreign vehicles coming into Mexico be inspected for permits, drugs, merchandise, and foods that are prohibited.
They took our permanent import papers (pedimento) and ran them through their system.
If it's good enough for the U.S. to do it why wouldn't any other country? It's not like we don't have enough troubles as it is.