A cellphone serves as a digital container into one's life. As it is a connected device, the cellphone has access to potentially limitless information about a person since the data is not necessarily stored on the phone itself but rather on servers elsewhere. Finally, one typically does not have complete control over the data that resides on their phone (installing the Facebook application, for example, allows others to stream their content directly to your device).

You're correct that this is unrelated to the NSA's surveillance programs. However, it does represent a threat to US citizens' constitutional protections.