The groups' original purpose was to conduct urban warfare and guerrilla actions in both defensive and offensive role, a mission for which they were trained by the US 9th Special Forces Group. The special units are usually deployed, several times every six months, in law enforcement actions that otherwise would go to elite police units. But since so many of them are in bed with the drug runners, there's essentially no one left to do the job. In fact, many of the units' missions involve chopping and burning down poppy fields, something that even the regular army and police would be capable of doing under constant supervision, instead of tactical raids and other jobs more oriented towards their specialized training.
All of this takes time away from training for war and other operations, but since Mexico isn't under any conventional threat right now, the government assumes its ok to use the units in its never-ending battle against the drug cartels. Also, the small size of the special forces groups makes them vulnerable and isolated in areas of the country that are virtually governed by narcotics gangs, especially considering the unreliability of the cops who would more than likely turn on the troopers if given the sufficient monetary incentive.
The US has a huge hand in training these units, as over 539 Mexican went through courses at the SOA (School of the Americas, which closed in 2000). Of these, 27 studied small unit tactics, three, psychological operations, 210 in training-related fields, and 268 in other specialties. Only 12 officers undertook any kind of specialized counter-drug training. The units are given what could be considered standard commando training: survival and combat operations in jungle and mountain terrain, urban close-quarter battle (CQB), raids and ambushes, as well as airborne and heliborne insertions. Thus, they are not prepared for the tasks they are given. In any case, special forces units are likely to be used continuously in Mexico's drug war for the foreseeable future, since neither the army nor the police seem to be getting any more honest than before. This by no means makes the units immune to corruption. More than 20 special operations troops have been arrested for drug trafficking over the years, but its a far cry from entire SWAT units being arrested at once for corruption.
Mexico's military special operations units are increasingly involved in the country's law enforcement troubles, having a significant impact on the units' ability to conduct the military operations for which they are intended. The drug war in Mexico has, to some extent or another, involved all elements of the country's law enforcement and military organizations, some with more success than other. The local and federal law enforcement agents, as well as the army, are well-known as having major corruption problems, with high-ranking army generals and entire counter-narcotics units having been arrested for aiding drug and people smugglers. As such, the country has, for years, called in Mexico's elite military units to help fight the drug war. The rationale is the same as it is in Colombia and Thailand: the militaries special units are less corrupt, so this makes them more suited to fight criminal activity. However, this rationale is only partly true. Mexico's 14 Airmobile Special Forces Groups, composed of some 1,400 officers and men, are definitely not as corrupt as other parties involved in the drug war, but police actions are not their intended use. The units are organized into company-sized units, about 1-2 units for each of Mexico's 10 military regions. At a lower level, organization is a lot like US Special Forces, with a higher ratio of officers to enlisted men. The standard team consists of about 12-14 men, with two officers commanding about 10-12 NCOs. Each regional unit has about 8-10 teams.