Illegal courtesies are when car drivers try to do something nice to bike riders, such as stopping to let them pass, but in doing so they are actually behaving illegally and dangerously.
BikeAthens has a short (under one minute) video on how to deal with illegal courtesies:
Anecdotally, I’ve observed illegal courtesies increase in my city as we built more bicycle infrastructure. Sometimes it feels like a “good” problem to have. After all, wouldn’t the world be a better place if everyone was too polite to each other?
(Source: Alphonse and Gaston on Wikipedia.)
Unfortunately, we can have too much of a good thing. These seemingly-polite actions create dangerous situations, and they force bicycle riders to make uncomfortable decisions. To accept such a courtesy means illegally biking into traffic, and to reject it means being rude.
When a car stops unexpectedly, someone on a bicycle usually isn’t able to tell why it stopped or when the car will unexpectedly start again. There may be other cars driving nearby, and there’s no telling whether or not they will also stop. Accepting the driver’s kindness puts the bicycle rider at risk of a crash.
Like the video says, people on bicycles should always reject these courtesies, even if it seems rude. The kindest thing that drivers can do is simply drive predictably and obey the traffic laws.