Not everything is evidence. When there is an email on the computer that will be used as a proof - then why is this been tampered with, if just a system log was

Not everything is evidence. When there is an email on the computer that will be used as a proof - then why is this been tampered with, if just a system log was altered? Courts are usually very strict to only allow "clean" evidence. Cases have been thrown totally over, because of a minor fuckup of police. It is all a question of how it is implemented and used. And sure, it is a very extreme thing and sadly the use case "only for serious crime" is in danger of soon to be applied to allmost anything.