If I am not mistaken, a cyclists has an increased heart rate and respiration. That means that the act of riding a bike results in greater emissions of carbon dioxide from the rider. Since CO2 is deemed to be a greenhouse gas and a pollutant, bicyclists are actually polluting when they ride.
Washington Rep. Ed Orcutt, defending a bike tax under consideration as part of a larger tax plan. He later apologized, calling his comments “over the top.” (via officialssay)