for some, the rule "no harm, no foul" could apply.
since alot of people ride bikes here in basel, many
cars are used to cyclists and pass really close. its
happens to me daily.

i think another thing is the more serious you look
on a bicycle, the more likely it is that a car passes
you closely. maybe most people think that the cyclist
who spends alot of time on the road is used cars
passing them.

another possibility is the people are just dicks and
are passing a close as possible to piss you off.

edit: just for the record, this sort of thing pisses me
off too ...i've just gotten used to it and usually don't
care enough to try to reason with a motorist about it.