I agree with your observations about the attitude of the people at the border. We finally decided that someone was targeting pets and refrigerators for their PhD project research. Some of their comments were so ridiculous that it was all we could do to remain polite and calm.
Our cat is very personable, and in the US, cats don't need rabies shots. When the vet at the border said we couldn't enter because we didn't have a rabies certificate for him, we verified with her that she was a vet, and offered to pay her to give him his shot. It went downhill from there, and I ended up getting a "warning." A business card size piece of paper that she made no copies of, that listed my name and the cat's name and said we couldn't enter Mexico again without a rabies certificate for him. It holds a place of honor in our Mexico treasures.
Yes, it's important to obey the laws, but when it's not the law, but for the "good of Mexico," it can be trying.
We find Sonoyta/Lukeville a great place to enter the US because of the lack of lines, and no automatic secondary for RVs, but we no longer enter there.