Because it is relevant to the conduct of the arresting officers: they didn't find him inside the house in the act of burgling it, they found him outside having been stabbed, with the 'perpetrator' (as they would have initially seen him) nearby. So, not a 'neat' 'home invasion', as you would have it
As they can here. But the law says they can only be questioned under caution (having been made aware of their rights), so they have to be arrested to safeguard those rights. They don't have to answer the questions, but their answers will only be evidentially admissible if they are under caution.So, you lock them up for breaking no law and interrogate them.
Just so you understand U.S. law a little better, we can only interview or interrogate a person with their consent. They can say don't want to talk, I am done talking, and it ends there.