Some folks are angered or bothered when they see someone trying to get fit. They are not actually being slowed down any, what you're seeing is a cowardly passive aggressive action being taken behind the cover of a moving vehicle. Little kids like to yell out of bus and car windows at people too but at least children have an excuse.
If you're stupid enough to don those gay shorts/jerseys/fakey plastic helmet and put yourself out there on a .01 horsepower machine in the middle of thousands of tons of steel moving 4-8 times your speed, then you are stupid indeed and deserve to get ran over. It's your right, it's legal, but it's damned stupid. I feel the same way about those ridiculously underpowered scooters.
From a somewhat-serious bicyclist in some form or fashion over the past 30 years: Riders in-town deserve right-of-way at all times and generally use good sense. The Lance Armstrong-wannabes, however - most of whom are brand-new bikers - riding outside of town tend to not use great judgement when choosing the roads they ride. These are the people Dean spoke of. But they generally very quickly learn the error of their ways (if they had sense to begin with)
I see it the other way around...in the country there are few, if any, alternate routes to take...now if there are in a group of 20 or 30 I tend to get pizzed... In town I have seen bikes on a major street during rush hour when they could have moved one block north or south and traveled side streets with no additional time or effort needed...but hell no, it is their right to drive on the busiest f'n street in town so they are going to do it...causing already aggravated rush hour drivers to become even more agitated...
Lets look at the real risk. Driving a car is about the riskiest thing you do every day. It is as risky as riding a bike. Maybe more so.