I think the logic goes like this:
Personally I still like the "compulsory" 1 minute water break to take things off the boil. I'll try it one day.
- You want to avoid a scrum to keep both sides apart to calm down a bit
- By awarding a PK to the defending side (ie the side in whose half play was) the chances are they're not going to score from it.
And if many of the players know perfectly well that you are wrong, they also know you are guessing (at best) and your credibility nosedives. If you have not seen a penalty offence (and one may have occurred during the part of the scrap you did see) then you have no sensible option but to restart with a scrum - and a dire warning that you will be watching for foul play.