I shall present to you two scenarios to illustrate what can happen when you click Punish or Pardon on a case in which there is only one punishable incident out of multiple incidents.
Scenario 1: Punish
First time offender has one punishable incident out of X incidents on his/her first Tribunal case. A majority of voters click Punish on this case.
First time offender receives a warning e-mail from the Tribunal system.
First time offender reads the mail and examines his/her reform card provided in the mail.
First time offender is not sure why he/she got warned and goes to the Tribunal forum to ask for help and insight on the matter; providing the link to his/her reform card for the community to look over.
The community points out what the offender did wrong in the one punishable incident.
First time offender is now more careful not to commit the same violation as he/she was pointed to on his/her reform card.
First time offender behaves better, resulting in pleasant matches in the future, and no suspensions.
Scenario 2: Pardon
First time offender repeats the bad behavior that would have been shown in the reform card, making several matches uncomfortable for over a dozen players that have been unfortunate enough to be his/her team mates.
First time offender returns to Tribunal, but because he/she has had his/her reported incidents mixed with false reports or borderline reports, first time offender gets pardoned again.
First time offender AGAIN repeats the bad behavior that would have been shown in the reform card, making several matches uncomfortable for over a dozen players that have been unfortunate enough to be his/her team mates.
First time offender returns to Tribunal, but because he/she has had his/her reported incidents mixed with false reports or borderline reports, first time offender gets pardoned again.
First time offender AGAIN repeats the bad behavior that would have been shown in the reform card, making several matches uncomfortable for over a dozen players that have been unfortunate enough to be his/her team mates.
First time offender returns to Tribunal, but this time is not lucky enough to have his/her reported incidents mixed with false reports or borderline reports because first time offender repeated the bad behavior from the initial reported incident in the first case a lot more as he/she came to perceive the bad behavior as "acceptable" behavior due to never being warned by Tribunal.
At this point, first time offender has been flagged by the system for audit, and receives a suspension manually administered by Riot staff instead of a simple warning.
A lot of players that had to put up with first time offender could have been spared the grief if the voters had punished him/her in the first case instead of letting him/her off with a Pardon.
First time offender might never have been suspended if voters had been thoughtful enough to click Punish in the first case to send him/her a warning about his/her behavior.