He was there "reasonably"~ so he didn't need a warrant. The focus of the court was primarily on whether or not the perp could be charged with an assault on the officer.
My conclusion is that the justices didn't read the final write up at all ~ this is a concoction by a power mad law clerk who's trying to show off to his buddies at the Turkish baths or something.
It's like the conclusions fit some other case ~ maybe ~ but certainly not this one.
Can you provide a link to the text of the decision?