Reading the second-hand article (now posted on CB as 3rd-hand article), it's not clear to me what exactly was requested and what actually happened.

My interpretation:
- The casket was transported in the cargo hold, out of everybody's view
- The official escorts accompanying the casket sat in the cabin with the rest of the passengers; they were probably in full dress uniform and likely sitting in the front of the plane per Delta policy
- The announcements asked for passengers to remain seated until the escorts had deplaned
-- The upset poster understood the announcement to mean, "Remain in your seats until the ceremony is over and the hearse has departed"
-- The passengers and crew probably didn't really know what the announcement meant; they had no idea what the ceremony consisted of. At least some of the passengers interpreted the announcement to mean, "Remain seated until the escorts have left the plane". They respectfully did just that, and then got up to leave. The rest of the passengers did the normal crowd/herd behavior and figured they must know what they're doing and followed along.

Sad, but not necessarily jerks or idiots.