Executing someone without a trial -- no oversight, no opportunity to defend one's self -- that does less harm to the rule of law than acting in accordance with the principles of due process as set forth in the Constitution?
The Constitution enshrines certain basic rights when it comes to criminal trials: the opportunity to confront your accusers, etc. All of these would have to be discarded to make a trial in abstentia happen. This creates the precedent: "if the person is bad enough, the Constitution doesn't require a 'real trial.'" This is a much more dangerous precedent to have on the books than "if you declare war on the U.S. and hide out in Yemen for a decade evading the reach of both American and Yemeni attempts to capture you, you might get killed by a drone."
It's better to just say that the criminal trial procedure doesn't apply in this case than to water it down to make it practical for this case. It's worse for Al Awalki, of course, but better for everyone else.
