Same applies here. If 10 officers misbehave, it is easy to fire them all, as you suggest.
If a majority of the entire police force defects, your only choice is between limiting the scope of the punishment to a few ringleaders vs. basically disbanding the police force and starting a new one from scratch, hoping that you can even recruit enough people to do so; but, in the meantime, the city won't be policed anymore, as the entire institutional memory has been purged.
In most similar cases in history, the authorities opted for a blanket pardon, as it is much less of a headache.
It is not even a new problem. Police is a relatively recent institution, but armies, gendarmes, legions etc. rebelled all the time, and peace usually had to be bought by concessions.
