This is every work environment. When there is a complaint, management is supposed to investigate to determine what is the cause of the complaint. When something goes wrong, management should examine that as well to figure out why. I would be much more concerned if I worked somewhere when, when things go wrong, nothing was done to figure out why.
And I don't understand how investigating everything before coming to a decision is 'guilty until proven innocent'. It's pretty directly not guilty until proven guilty.