I think the quoted the wrong post, but here I will say reasons are going to vary.
I don’t know. Moderation in all things I think. If you aren’t engaging enough as a GM, distractions can arise and that is instructive. But people are also human so there will be moments in games where, if you are playing online, some might succumb to a temptation to check e-mail or something (it need not be combat, it could be the party splitting up or anything that gives other players reason to not pay attention to what is happening). I don’t think playing D&D or D&D like games is the issue. I play lots of different games and there will always be moments where someone isn’t as focused as others. The real issue is if this becomes a problem that affects the game. My view is, I am not here to tell people how to conduct themselves. If someone has a habit that gets annoying there may be a conversation at some point of course but otherwise being a bit tolerant of normal distractions arising during a game is more a sign of maturity to me than overreacting to someone checking a text from their wife or having to check on something in the other room. One side effect of playing online is people are in their homes still available to the people they live with and all that is more important than a game. When people take the game too seriously, that is a bigger indication of emotional immaturity