A lot of the reasoning behind extending surveillance stems from the need to find the people who enable the dangerous people to do dangerous things.
One example: when I got robbed a couple years back (an armed guy stole my laptop), I had the opportunity to discuss the strategies with the detective in charge of my case. A lot of resources were devoted to catch the guy with the gun, but very little to catching the person (or organization) buying the stolen goods (in this case, a laptop) and reselling them. If you catch the robber, it's easy to replace him. If you disrupt the chain at the receptor, you will do more damage. OTOH, if you catch the unsuspecting buyer of a stolen laptop, he (or she) will gladly point the authorities to the store where they'll find a convergence of many such value-chains. This is where most of the money is and where the most damage will be done to the system. That's why now I have the serial number of my laptops written down and all their labels photographed and stored. And all sensitive information encrypted, in case they don't want my laptop, but the data on it.
Having said that, catching the people who support the really dangerous extremists, the drug-dealers, the pedophiles and the slave-traders involves catching who, at the surface, seems rather harmless, making donations to religious organizations, smoking a joint at a party, buying porn online and groceries from Walmart.
On one hand, we may want to make our technology difficult to abuse, but, on the other, we may also want to find people who are very good at protecting their tracks, and do so through people who really don't know how to do it.
Not too be too difficult, because I'm actually interested in whatever rationale behind extended surveillance exists, but can you cite something official for me to read about this "reasoning"? Because it seems to me that much of the rationale for ridiculous amounts of surveillance is "Hey! Terrorists!" That's certainly what's behind the War on The Unexpected, as manifested by the TSA and DHS.