This One's For You, Mr. Rides-Bicycle-In Street

Yesterday, while driving, I came upon a man riding his bicycle.
He started off on the sidewalk, but then for some reason, merged onto the street.
I didn't quite understand this because a) the sidewalk goes on for quite a ways past that point, and b) I didn't understand why he felt that this was a good idea, he definitely could not pedal as fast as the cars were going.(I don't think many people could, unless they're Lance Armstrong or something).
Now don't get me wrong, I'm all for people exercising and enjoying fresh air, especially when it's wonderful outside, like it was yesterday, but this man just really annoyed me for some reason.
He KNEW there were cars behind him, that had to slow wayyyy down, and he wasn't exactly riding in the middle of the street, but more on like the passenger side of a car. Well, after riding behind this guy for a few minutes (which scared me because I was afraid he was going to fall off or something) I decided to pass him (which scared me even more, because I was afraid that he would decide to move over). The car behind me decided to pass him too.
I am just extremely puzzled why someone would decide to do this, because it seems very dangerous to me (because it was rush hour and he was riding down the main drag in town) and because of where we live...our town has a very large greenway, a park, a nature trail, and (if you just really have to ride on the real streets) many many subdivisions that don't have much traffic at all.
And, as I mentioned earlier, the street also has a sidewalk. Ordinarily I am definitely not a fan of people riding bikes on sidewalks. For example, on the campus of the college that I attend, I become greatly annoyed when I am almost run down by someone on a bike, but bear in mind that the speed limit on campus is only 15 mph, and I feel that it's easy to ride in the road and still be very close to the sidewalk (and out of the way of traffic) without actually being on the sidewalk.
But, in this case, I do feel that it would have been better for this man to have just stayed on the sidewalk, since the street was so busy and no one else was walking on the sidewalk anyway. I really want to stress that it was not the inconvenience of having to slow down that annoyed me the most, it was the fact that I feared for the man's safety, and the fact that there are so many safer options available where we live.