Idiots use Webmail
Because most of the world has gone to webmail, especially Gmail. You don't need an e-mail client then. Fewer programs to maintain, fewer potential exploit avenues and/or points of failure.
I know the people involved in this episode. From the "evidence" the government initially presented, it's clear it all came about because the University had, like many organizations, adopted Gmail as their university email system. And the US government read everything, misinterpreted it, and arrested the guy.
Remember that when you use an email service you don't control.