But the fact that they're mailing tiny samples to a few news organizations implies that they're trying to get maximum effect (publicity-wise) out of their small stock.
This makes sense -- the terrorists are acting in the way they'd be forced to if they only had a small bit of anthrax powder to use, and wanted to make the most of it.
Agreed. Mailed to the news outlets where they could not ignore the story. Maximum publicity from minimum supply.