Mexico's Constitutional Law states individuals are presumed guilty until proven innocent. Mexican Law Enforcement Officers have the right to deny anyone they feel are in violation of the law. They have the right to pull you over, search your vehicle, question you, impound your vehicle, hide / arrest you, send you to jail, and hold you until arraigned without sufficient probable cause. They can do this because you are presumed guilty until you prove you're innocent.
How does that apply to auto insurance? If a law enforcement officer notices you have broken a law while operating a vehicle, you are presumed guilty. Once a claim is filed with yours or the other party's insurance company, it is the responsibility of the insurer to determine whether to overturn your guilt or leave you on your own for legal representation. Example, you and another vehicle have a fender bender with no clear evidence as to who was at fault. At that point, the law enforcement officer has the right to arrest both parties until arraigned in front of a judge. The judge can then rule as to who is at fault. US Auto Insurance companies are not prepared to offer liability coverage for this occurrence.
US Insurers offer liability coverage with the assumption one of the parties involved in the accident are at fault, not determined by a court. US Carriers offer legal representation to their policy holders if a claim or suit is bought against them as a covered loss, stated in the insurance policy. If the legalities of the loss are dictated by a court, Carriers are not willing to offer legal representation because if they are defending the guilty party, they are spending more money than allocated in the grouped premium. Mexican Insurance carriers are prepared to offer liability coverage because their policies are designed in conjunction with Mexican Constitutional Law.
Word of advice, when traveling south of the border with a vehicle, get Online Mexican Travel Insurance from one of several well established carriers. Most of the Online Mexican Travel brokers offer the same carriers such as GNP (oldest), ACE, and GE and everything you need to quote, bind and issue coverage is readily available on the website.