About finger on the trigger...

Do law enforcement and military do that? Keeping finger off the trigger when they could easily get shot while clearing a room they've been already fired upon? I thought they only do that when they haven't encountered the enemy yet and/or that the enemy haven't posed any hostile threats.

And not to forget the classic I can take this guy down and arrest him myself even though he's kill many of my fellow officers only to get killed himself without waiting for backup shtick.