It is if there is no indication that you are replying to everyone though.

Disclaimer: I don't have a fancy phone so I don't know how clear things are.

Just because something doesn't work the way you want it to doesn't mean it's a bug.

True, but an interface that you tend to use for messaging to one person should give an indication that you are, instead, messaging to several.

True, a buddy of mine posted something as a Facebook message instead of a wall post and every reply went to everyone on his buddy list. Soon it was people complaining about getting messages at late hours. Apparently you could 'unsubscribe' from messages on the website but not on the mobile app.

To this day I'm tempted to try replying to the thread to see if will still go out to everyone.