I'll tell you exactly why they would target the Enquirer. Becasue next week every supermarket in the USA will scream hadlines of TERRORISTS ATTACK US WITH ANTHRAX.
Nothing like a little mass hysteria for all of the common folk who pick up these rags at the checkout lines.
Are you suggesting that Stevens killed himself for publicity? It hardly seems likely.
It MAY have been Atta. These are the kind of guys who bear grudges, and if he was offended in some way he might have taken the chance to get a little personal vengeance around the same time he was doing the big, official job.