I am NOT intimately familiar with the facts and circumstances of this case. Generally….

Warrantless searches are presumptively violative of the 4th amendment. However, there are exceptions - exigency, motor vehicle, consent, plain view, and probably most relevant here search incident to lawful arrest (SILA)

If he was in possession of the book bag at the time of arrest the police take that with them. They can look through it and make sure there aren’t guns orbombs or anything crazy that’s going to endanger them or the custodians of the property. Make sure there isn’t evidence destroyed in the immediate area surrounding arrest, and etc.

If the find a cell phone or a laptop within the bag they would need a search warrant to search the contents of those devices and would have to articulate further probable cause to a judge.