The problem is that players need reinforcement to treat a ranked ladder game seriously. If you are not willing to commit to playing a game where not only your rating but the rating of your teammates matter, then you should not enter Competitive Play.
Unforseen circumstances are always possible to any player. That is why the SR penalty is fixed at 50 SR per leaver violation and the starting suspension is only 15 minutes. A just right amount of time to get a player’s attention and work to resolve issues before they attempt to play again.
Skill rating is a culmination all of your matches including the matches you won due to a problem with a player on the opposite team. The fixed penalty for SR is a visual deterrent which reinforces the idea part of your skill includes your ability to actually complete the games.
The rule is that if you leave or disconnect after you participate in a match, you have two minutes to return to that match without penalty. If you fail to participate (deal damage or healing) at the start the match will cancel and you will be penalized. If you fail to return within the two-minute window, you can still return, but your personal result will be marked as a loss and the penalty will be applied regardless of the match outcome. If you fail to return by the time the match ends normally, you take the loss and penalty regardless of the match outcome.
Again the leeway is the small penalty. Otherwise players would not take the time to fix their persistent issues.
Details of Blizzard’s policy regarding tech issues and why penalties are still applied can be found in this forum post: