...because they don't need a warrant. A warrant is needed when the state wants to use its monopoly on violence to obtain evidence (eg. sending men with guns to search your house). It's not needed when someone volunteers the information (eg. asking your friends what they saw when they last visited).
"Such records can identify Americans who are seeking help from a suicide hotline or a hotline for survivors of sexual assault or domestic abuse,"<p>Anyone have a good hypothesis about why the NSA would want this specific data? Only good thing I can think of is to look for folks in their ranks who are at risk of being compromised.