I'm glad I was able to find this, because I was beginning to question whether I was going insane since everyone else seems to assign people to lower levels without any issue, yet I recall getting hit with the demotion penalty.
From the 7/5/07 dev. chat:
settle a recent debate for us: we know that if you demote a plyer, there's a risk of a demotion penalty depending on various factors (age, makeup, patience, etc.). if you assign a player for whom you just traded, does the same risk apply? i'm not talking about that player being stuck at the new level for a long time, i'm only asking whether assigning a newly-acquired player to a lower level functions the same way as a demotion. thanks. (schedule1 - Hall of Famer - 4:13 PM)
Assigning a player to a lower level can have the same affect as demoting a player.