The reason that is doesn't fall under warrantless wiretapping is because there isn't any wiretapping going. The data that is being collected by the NSA isn't the actual conversation, but the metadata of that conversation (location, duration, etc). This is because in almost every case the conversation that is going on is unimportant and information that is most interesting is who people are talking to. This information is important because by comparing all of the information they can build a graph of the various social networks which is particularly important when tracking terrorist networks. And by racking who is talking to they have a better understanding of the phones that they need to get warrants for and whose phones they need to tap (this is actually how the NSA figured out how the bin Laden network was operating). Thats as much as I can figure out, because its illegal for the NSA to tap domestic phones. Anything domestic tapping comes under the FBIs jurisdiction.
I wonder what is the expectation of privacy regarding phone logs. Knowing who I call, when I call them, and for how long is potentially very significant private data. It can be evidence of an affair, or evidence helping to convict or acquit in a trial. It could also be used to target all kinds of networks, including political ones.