So, signature strikes are those that identify 'targets' by watching for things like people who leave militant camps with guns to go cross over into Afghanistan and then come back again...

Read up on the situation in Pakistan's tribal regions before setting your position on drone strikes in stone. America is at war with the powers controlling the tribal regions of Pakistan. It's not declaring it a war because that would enrage Pakistan even further, but it is the reality. Pakistan doesn't want to admit that the tribal regions have been essentially seized from them by the Taliban and have been in all but name an independent state that is actively waging war on Pakistan by launching endless attacks on it's civilians.

The deeper truth not spoken about is that the Afghan war was never about Afghanistan, though it was about the 9/11 organizers. After 9/11, America deemed it no longer acceptable for nuclear armed Pakistan to be allied with the groups that just attacked it. The war in Afghanistan was double edged. It gave a launching ground for a land war against Pakistan if needed, and it was a direct warning to Pakistan's leadership that America fully intended to wage war and remove power any government allying itself with the jihadists behind international terrorist attacks. It's just not politically expedient to say that, so president's say more veiled things like: "Your either with us or against us".