A.P. reports that "Mexico's new drug use law worries US police." Specifically, San Diego Police Chief William Lansdowne says now that possessing small quantities of drugs is no longer subject to criminal penalties in Mexico, college students "will go because they can get drugs." San Diego County Sheriff Bill Gore likewise warns:
It provides an officially sanctioned market for the consumption of the world's most dangerous drugs. For the people of San Diego the risk is direct and lethal. There are those who will drive to Mexico to use drugs and return to the U.S. under their influence.
Since it remains illegal in Mexico to sell any quantity of drugs or to possess more than small amounts (e.g., a fifth of an ounce of pot and a few lines of cocaine), these concerns seem overblown. Americans might be more inclined to use drugs in Mexico now that they no longer have to worry about getting arrested (provided they stay below the legal limits), but getting the drugs won't be any easier (unlike, say, in the Netherlands, where the government tolerates the open retail sale of cannabis). It's simply inaccurate to say the new law "provides an officially sanctioned market." As for stoned drivers coming back from Mexico, if they replace drunk drivers San Diego might see a public safety improvement, since marijuana does not impair driving ability as much as alcohol does.
This A.P. article repeats a contradiction I've noticed in several other stories about decriminalization in Mexico. It says one aim of the change is to "make room in overcrowded prisons for violent traffickers rather than small-time users." Yet Mexico's attorney general says drug users were very rarely prosecuted, let alone imprisoned, for simple possession under the old law. They were, however, vulnerable to shakedowns by police, who extorted money from them under the threat of arrest. The new law means police can no longer follow through on such threats, so it should reduce harassment and petty corruption.
[Thanks to Abdul for the tip.]