I don't know for sure, and maybe criminal courts do things differently, but in a civil motion, comparable cases would be cited to show the court that the facts in this case are like the facts in those cases, and therefore, why the court should find that the law should be similarly applied here.

With my limited knowledge, I'm not convinced it's a chain of custody issue, officers search people and things at the scene every day.

I also don't know if a warrant was necessarily required. Officers perform searches of purses and backpacks every day, and not just because they're looking for evidence of a crime, but also under theories of officer safety. Kinda of an extended Terry frisk.

If he hasn't yet been arrested and they didn't yet have a reason to seize the backpack, and the backpack was near him, that would increase justification to search it.

If on the other hand, they searched after the arrest, they could use the "inventory search" exception — that they were taking inventory of the contents for administrative and liability purposes.

I don't know if a warrant was per se needed here. That's why I think citations are needed, and at least to explain why the common exceptions don't apply.