Writing at Foreign Policy, Dan Drezner argues that drawing out the killing is the whole point:
To your humble blogger, this is simply the next iteration of the unspoken, brutally realpolitik policy towards Syria that's been going on for the past two years. To recap, the goal of that policy is to ensnare Iran and Hezbollah into a protracted, resource-draining civil war, with as minimal costs as possible. This is exactly what the last two years have accomplished ... at an appalling toll in lives lost. This policy doesn't require any course correction ... so long as rebels are holding their own or winning. A faltering Assad simply forces Iran et al into doubling down and committing even more resources. A faltering rebel movement, on the other hand, does require some external support, lest the Iranians actually win the conflict. ...
This is simply the United States engaging in its own form of asymmetric warfare. For the low, low price of aiding and arming the rebels, the U.S. preoccupies all of its adversaries in the Middle East.
--Conor Friedersdorf, The Atlantic, on the new proxy war