Why is Anthony Weiner in FMC Devens?
The media answer is because he entered a sex offender treatment program.
My guess is to keep him alive, because he made a deal and is a witness. If the rumors are true about his laptop, then he's gonna need to be kept alive long enough to serve his purpose.
Link about his treatment
http://www.nydailynews.com/news/politics/anthony-weiner-set-sex-offender-treatment-mass-prison-article-1.3586030#
Link about psychology intern program at prison.
https://membership.appic.org/directory/display/187