[pliny]

>What's the justification

It's along the lines of - if we're monitoring a service to catch terrorists or prevent terror attacks, it's counterproductive to alert the terrorists they're being monitored. Furthermore - if the people trust us enough to allow us to monitor these services (which is supposed to follow from there being laws enabling us to monitor these services [a]) then you must also trust us not to abuse the information we gain or to use it in any way that does not serve the interest of national security, so non-terrorists using the service aren't harmed by not knowing they're being monitored.