And I am not making accusations, but frankly it is really, really hard to find another explanation why this operation was done.
We do know that the tactic was not successful, and that there was no coherent plan for actually tracking these guns except to see if they showed up at crime scenes, which seems rather too late to be helpful in preventing crime, much less preventing guns flowing across the border.
It's plausible but so far unproven. We do know the administration had a motive. They started out in early 2009 trying to reinstate the Assault Weapons Ban using the Mexican cartel situation and the 90% lie as justification, before being forced to withdraw on those efforts. And we have the president telling James Brady that they were working on gun control "under the the radar".
It isn't totally irrational to suspect that a program that sent huge amounts of guns to Mexican cartels, with no means to track them until after the fact (thus, not a program oriented toward prevention), by an administration highly desirous of new gun control measures, might have been arranged one way or another to provide more of the evidence they had already been trying to use to make their case for increased gun control.
It isn't even necessary for them to have intended a bunch of guns would be lost and a bunch of people would get killed; that, per Heinlein's razor, can be explained by incompetence.