In a high profile murder case like this they will subpoena every possible contact, it's not really that outrageous amount in this situation, with the school, gym, shopping places, other high-density locations, etc, and (I think) they (the recipients) are also from that point limited on talking about the case, evidence, like a gag. Also, they can check out potential witnesses this way, and sort of gather up evidence, stories and hone a list of whom to select later to testify at trial.
IMO
LAWYERS: Help us out here on what *boundaries* a subpoena sets, por favor.