Much of the controversy is about whether it was a legal method of take, particularly if it is lawful to lure an animal off of a reserve by baiting and whether it is legal to shoot a collared lion.
Poprivit's joke about the collar notwithstanding, I cannot find a single picture online of Cecil where a tracking collar is visible. Big maned lions must have a narrow small collar which is much smaller in diameter than the mane so it is very possible that the collar was not evident.