Firstly, there are a total of two drones which are being deployed over Libya. This is hardly an escalation, especially considering that the US has around 50 drones in total. Also, given that the drones are being deployed for a very specific purpose (distinguishing between civilians and Gaddafi loyalist combatants), the intent of using them clearly is not the kind of overarching change in the state-of-play that an "escalation" normally would entail.
Secondly, Secretary Gates stated at a press conference earlier today that the option to use drones was recommended by the DoD itself. Given the DoD's open reticence to getting involved in Libya, it would be inconsistent with the Department's own objectives to recommend escalatory steps in Libya.
Ultimately, it appears that in deploying the drones, the U.S. government is balancing its humanitarian concerns for Libya and desire to show commitment to NATO and its European allies with budget shortfalls, an overstretched military, and a reluctant American public.