Comment 2 for bug 513803

From the schroot.conf manpage:

   Plain and directory chroots
       Chroots of type ‘plain’ or ‘directory’ are directories accessible in the filesystem.
       The two types are equivalent except for the fact that if run-setup-scripts is set
       to ‘true’ for ‘plain’ chroots, filesystem mounting is disabled.

Therefore I believe schroot is functioning as designed.