Technicality. They still are snooping where it is not their business.
Back in 1979, the U.S. Supreme Court held that a pen register is not a "search" at all for constitutional purposes, and thus requires no warrant, because it uncovers no information that the caller didn't voluntarily disclose to the phone company. Congress put some limits on the use of pen registers by statute in 1986, but it is far, far easier for the government to use a pen register than for it to get a wiretapping warrant.