Unfortunately, it’s a rule to ensure abusive players don’t attempt to hi-jack the match. If you leave/disconnect from a Competitive match, you must rejoin within 2 minutes to remain qualified. More details about Blizzard’s policy is here.
Honestly, I still try to rejoin, because I know my five teammates don’t need to be punished if I can help it. Yeah it sucks knowing, the penalty would still stand for me, but I also know my internet and computer is my responsiblilty.