This has been answered many times.
In the very first hands on preview, they noted that the people who were chasing Niko soon got pursued after an officer assumed that people were harassing a civilian.
I am back! My internet was capped so I couldn't get on. I can't believe I missed out on trailer 4! Well at least this month I'll control my downloading to see IV's release.
Urbanoutlaw they did mention this. Niko was being chased and the cops assumed the enemies were harassing a civilian. I think it was the first hands on preview that they made the mention of this.