Sportsmanlike conduct is not about what mode you are playing.

Being a sportsman, and sportsmanship is how you conduct yourself all the time. It isn't about what mode you are playing, what you feel like that day, or anything else.

If going into a teammate's lane and taking cs is something that is unsportsmanlike in ranked, it is unsportsmanlike in co-op v AI. Sportsmanship is about your character (who you are when nobody is looking). If you aren't being sportsmanlike all the time, you aren't sportsmanlike ... it isn't something you switch on and off.

If you disagree, I'd highly suggest you examine yourself. Are you making excuses for how you treat other people?