The starting penalty for leaving a Competitive match is 10 minutes suspended from the Competitive Play mode and a full 50 SR loss. This suspension time increases if the player continues to leaving multiple Competitive matches. It can take as little as six consecutive leaver violations to trigger a season ban. A player who is banned from the competitive play season will forfeit all end of season rewards including bonus competitive points. Furthermore, a player who is banned from three Competitive Play seasons will receive a permanent ban from Competitive Play.
The starting penalty is relatively small but firm because Blizzard knows that many players do not leave a Competitive game on purpose, but they have disconnected or crashed in some manner. Disconnections and crashes are no fun and getting penalized for it can feel like salt in the wound, but by penalizing all forms of leaving including those caused by technical malfunctions. Bad leaver behavior including being ignorant of technical issues can be corrected. On a side note, Blizzard will never use increasing skill rating penalties or make the leaver take the SR loss from the rest of the team because this would give players an easy way to deliberately tank their rank in order to smurf lower rank matches.
The remaining team members take a loss because if there was any form of compensation, some players could try to become disruptive and bully other players into leaving the match. This was a bad problem back during season 1 when the game used to compensate for when there was a leaver.
With all of this being said, I did recently confirm during the Reddit AMA with Principal Designer Scott Mercer that the Overwatch team is looking to revise key rules for leaving games in Overwatch and we should be expecting changes sometime soon.