But now they're reporting they found plastic ( http://nasaupdatecenter.us/press.html ) which on Earth at least is created by petrochemical compounds caused by... life.
If nothing else, we know "where" to focus our search for extraterrestrial life.
I fully expected this is to be something like that despite their clean-room standards of handling the rover a bit of hair or skin did wind up in the thing somehow.
Why even say anything in the first place?
To quell expectations. Nothing is worse than announcing that you have found something very interesting (scientifically), only for the press to report "NASA doesn't find aliens, should funding be cut?"
"in the first place" would imply that they don't make the initial announcement, so there would have been nothing to quell.
They haven't (or at least hadn't) made any announcement. The only announcement was for a press conference on December 3 to give an update. The rumour mill provided the rest.