I think a big part of it is that presidents don't really think for themselves. They are essentially at the behest of their advisors and the heads of these vario

I think a big part of it is that presidents don't really think for themselves. They are essentially at the behest of their advisors and the heads of these various organizations. If the head of the NSA tells you that the program is constitutional and we have X, Y, Z safeguards in place to protect Americans, why would he not believe it? It's not like he can evaluate the implementation himself. I doubt he would even have time to if he wanted to dig into the gritty details. The fact is, there is a machine that exists beyond any sitting president, and that machine is adept at getting its way. Convincing one person, who does not have any former experience in these matters, that your way is the right way, is probably the easiest part of their job.