The problem with spying on everyone is that you get too much data to process properly. Even if the US intelligence apparatus managed to gather data that would lead them to an elaborate terrorist plot to subvert a few key flights and crash them into nasty places, how do they differentiate this data with the millions of other possible threats they deal with every day?
As an aside: if you're actually surprised that the USA spies on everyone, both enemy and ally, you're more naive than you deserve to be. If it were my job to run any sort of country, knowing what both my allies and my enemies are really thinking and really doing would be at the top of my priority list. Most countries run multiple intelligence apparati for both domestic and foreign affairs.
In fact, as the USA, I would suggest that there are many parts of the world where we need more spies and surveillance, not less. For example, we could use more spies reporting on the actions of our private contractors in Iraq. I'd like to know exactly what they're doing with the money the US military is paying them, and what else they're up to on the side. There's a lot of shady stuff being reported in that sector.