We first started discussing this seatmap issue a few days ago here:
http://www.flyertalk.com/forum/delta...ap-issues.html
It started with new functionality that was rolled out overnight from Wed to Thurs morning that enabled the ability to upgrade individual segments.
As others noted, you can only see the seatmap in the cabin you purchased - except if there is an FCM offer, then you can see everything. My guess is a glitch/side effect, since it really makes no sense why DL would block viewing seats in other cabins. It has no real benefit to them, and it just is an annoyance to pax.