Because of rules. It could not be clearer:How can those two offences be treated the same way?
10.6. With the exception of the Masters’ Divisions, and at half time, in Divisions 2, 3, and the U21Division, a team proven to have made more than five (5) substitutions in a game shall be considered to have played an ineligible player. The provisions of Article 6.5 of the By-Laws and Section 8.b of the Rules and Regulations shall apply.
The definition and punishment for the infraction are explicit and were followed in this case.
If the teams/membership feel that this is not the appropriate way to deal with this infraction they can propose the rule be changed at the AGM and then it will be voted on.
In the meantime, this one is pretty open and shut.