I think it was just a case of first appearances don't line up with what perhaps we Americans think of as police motorcycles. In the US, there are commonly clad in typical US police colors, usually variants of white, black, and blue.
I don't think yellow is common in the US for police vehicles of any variety. To me, they certainly didn't scream police until that second photo showed police near similar bikes, and then I saw the tiny text that said "Police" and noted that the text was also on the bikes in the original photo.