Collecting information prior to the delivery of a warrant is unreasonably invasive. If you extend the reasoning then they could post a video camera in your home, filming everything you do, and then get a warrant to obtain the material at any point in the future. Our digital homes should have the same safeguards in place that our physical homes have: nobody has access before a warrant, and with a warrant they can only find out what's there now, not what's been there in the past.
Are you referring to Prism or the "beam splitting"?
Prism sends over data that was already generated; it's not the government's fault that the company bothered to generate it but if they get a warrant why should they pretend like it didn't exist? I'm not sure how you would even begin to investigate things like 9/11 if you weren't allowed to collect evidence from actions that occurred before the crime came to the attention of the government.
If you're talking about beam-splitting then, well, I see your argument. But that's not what I was talking about in my parent comment.
