I think drivers just honk because many feel bicycles should be on the bike paths and not on the road, even if the cyclist is abiding by the rules of the road.
I was riding in a small town in W. Mass a couple weeks ago where the speed limit was 25mph through the center of a small village. This area is a notorious speed trap and since there were cars parked on both sides of the road I had no choice but to ride in the lane for maybe 1/8 mile. According to my bike speedo I was exceeding the speedlimit (27 mph) and this B***h in a Matrix HONKS at me (there was no on comming traffic and she could have easily passed me despite it being a no passing zone. Anyway as she sped by after I was able to move over once I got past the area with cars paralled parked on the side of the road (these weren't parking spots either just the shoulder of the road), I yelled "The Limit's 25" and she gave me the finger! Oh well normally I never get honked at...