It's actually cluelessness, but in a good way. They know ahead of time that they don't know which items might be important evidence. Law enforcement officials collect everything and sort it out later. It's really hard to go back for something that you find out you should have collected but didn't.

Seizing equipment is not just for digital evidence either. There might be physical evidence. At the beginning, you don't always know what is important and what isn't, what belongs to whom, and so on.

Your analogy to the murder victim is invalid though. Law enforcement and court officials will have access to the evidence. Also, a coronor won't necessarily not do parts of an autopsy because someone already told them what happened. Examination does not only support a story, but also rules out other stories.