Can't sacrifice Druids?
 

I found this weird bug or feature while supressing the Cloak of Wizardry. I left one friendly Gremlin tower intact to summon creatures I can hypnotize and sacrifice... to my surprise, the cursor turns into blank when I ran it over the hypnotized druids.

Is it maybe because they are considered to be summoned units? I found that I cannot sacrifice summoned demons either...

While you can use Sacrifice on Hypnotized enemies you cannot sacrifice summons, be they your own summons or the enemies' summons.