From http://www.nytimes.com/2013/07/18/opinion/the-drone-that-kil... : "The missile killed him, his teenage cousin and at least five other civilians on Oct. 14, 2011, while the boys were eating dinner at an open-air restaurant in southern Yemen...The attorney general, Eric H. Holder Jr., said only that Abdulrahman was not “specifically targeted,” raising more questions than he answered."

I don't know if it is common for u.s. army to kill people without targeting them but if it is the case they should better learn how to aim.

Almost every drone strike kills several people - usually only one of those people is the intended target. In many cases (including the one linked) they have admitted that the people killed were not the target. Not as an apology (those are rare, they don't feel the need to admit mistakes because of course the missions and targets are secret), but simply to say that those people were not the intended target, just collateral damage. Here's another well-known example of firing on a crowd:

http://www.theguardian.com/world/2002/jul/03/afghanistan.luk...

By the time you're firing missiles at a restaurant, you have accepted that as long as your target is killed, some innocents will die with them. Any civilians, waiters, passers by etc who are caught in the blast are clearly just the cost of doing business. The attacks don't happen in a war zone or under cover of war, except the eternal and ill-specified 'war on terror'. If you want to argue that is not specifically targeting innocent people I'd respond that they are indiscriminately targeting innocent people in the hope of hitting their targets, which is worse.

It's remarkable to me that people still try to defend these attacks - if missiles were used on targets in a town on the US, killing several bystanders, there would be outrage, and rightly so. For the civilians on the ground, it is equivalent to suicide bombers taking out a crowd, some of whom are off-duty troops - the two acts have very similar consequences.