This is so strange! When was a last time AP released anything that wasn't more than government PR. On many important issues like lead up to Iraq war and now to a potential Iran war, AP has just reported what's been expected of them.
My thought is the wiretap was set to catch potential whistleblowers than to do anything with reporters or AP itself.
As far as I can tell this was done via subpoena (but it's uncertain under whose authority the subpoena was issued, it may not have been a judge or grand jury).
Had there not been a subpoena the U.S. would likely not have informed the AP of the search at all.
It's not strange at all. If the AP's sources just disappeared, the word would be out to any future sources/marks that the AP gives up sources...but if the big bad government did this rare, but effective overreach...the spooks can still terminate the leak and the AP has plausible denial.
The CIA wants to maintain the AP's ability to lure in more leaks in the future.