According to the premier league's website "Any goal scored or created with the use of the hand or arm will be disallowed this season even if it is accidental", which I think is the basis on which most people have accepted yesterday's decision.
https://www.premierleague.com/news/1263332
However at the bottom of the same page is a link to the IFAB handball law changes in full which has the following definition...
The following ‘handball’ situations, even if accidental, will be a free kick:
- the ball goes into the goal after touching an attacking player’s hand/arm
- a player gains control/possession of the ball after it has touches their hand/arm and then scores, or creates a goal-scoring opportunity
- the ball touches a player’s hand/arm which has made their body unnaturally bigger
- the ball touches a player’s hand/arm when it is above their shoulder (unless the player has deliberately played the ball which then touches their hand/arm)
http://static-3eb8.kxcdn.com/documents/786/111531_110319_IFAB_LoG_at_a_Glance.pdf
- the ball did not go into the goal after touching Laporte's arm
- he did not gain control/possession of the ball after it touched his arm
So why was the goal disallowed?