Krishna initiating contact when he's in front and running towards the ball?
The defender runs behind and across Krishna and his leg inadvertently knocks Roy's foot, which causes him to trip. That's interfering with an opponent. It happens many many times in a season and is always penalised. This is only different because it's ended in a penalty rather than a free kick.
As I see it, Alessi running behind Krishna and causing him to trip is 'acting without precaution' - which doesn't require Alessi to be making a challenge to be penalised.
Yep, Krishna's foot initiating the contact can certainly be argued - at least as much as that the defender 'ran into him'.
The rest of your analysis is...well, I guess there's no need for insults. Suffice it to say it is very self-serving, and appears not to be grounded on the actual application of the laws of the game, or indeed the way the games is played. I would just caution you to reconsider what you've suggested - running behind a player is, according to you, acting without precaution, lest it result in contact with the opponent which may cause him to fall. That's not football, that's netball.
And as for 'this happens all the time' - I clearly remember an almost exact replica of this situation in an El Clasico from a few years back, when C. Ronaldo was told in no uncertain terms to get up and play on (I can't remember whether it was Mascherano or Pique in the defending position).
I'm not trying to be self-serving. Of course I'm not saying running behind someone is in itself acting without precaution. But running close enough that you trip the other player is. I also don't agree that Alessi and Krishna were running the same line - Alessi was running across the back of Krishna.
I don't know the example you mention but could it be possible that the ref didn't see the contact from the defender and thought Ronaldo tripped over himself?
Edit: This one?:
To me that also looks a dead cert penalty, and also think Mascherano deliberately shouldered him so it's different anyway.
Sorry to those who aren't interested in this but I do find this kind of thing interesting. Especially how people who love the game can have such varying opinions on something.