I mean, not that I "hope" it's an off-ice issue or event, but that seems like a more plausible and justifiable reason to take a leave of absence from the team, vs. "I'm not playing well so I just want to take a break".
With the kind of heart & soul, play through everything, kind of guy JT Miller is, it would take a massive wake-up call for him to say I need time away. From his perspective, he likely thinks he can help the team even at 50%.
With mental health though, sometimes you need to hard reset yourself vs. trying to always push through and deal with it. To me, the benching was the only real way someone like Tocchet could tell him "look, what you're trying to do isn't working is not helping right now, you need to take a break" without it just coming across as empty words.
I've had to coach a lot of kids through mental health struggles, and each of them required a different approach based on their personality & temperment. Needless to say, I'm not surprised Tocchet took this path with someone like JT, whereas he might be a bit softer with other players (like Petey).