1) Constitutionally they (NSA) seemed to have found a loophole that states that just storing the data on the disk doesn't constitute spying|invasion of privacy. Only when someone (a human) looks at the results then it triggers all the Constitutional restrictions. Sounds like a bunch of bullshit to me, but that's how they are justifying it.
2) A court subpoena or an executive "magic-Patriot-Act-Federal-Injunction whatever it is called?" when issued can apply to all the data, including historic data from the beginning of time associated with that individual. So, if they ever get a subpoena say when you are in your 50's they could legally pull all the data you generated, created, accessed since the day you were born.
Basically it is pretty obvious they are just planning on storing all the data they can. Therefore the big new data center with a 65MW power station next to it.
My hope is only that someone who is involved in this, just like this whistle-blower, will realize that this is wrong and will expose it and the public in general will start caring enough about this to turn this into a major political issues.