Another angle is that the cyclist knows that he is slowing down a caravan of cars behind them and yet he's still OK with that. It's the egoism that is irksome.

(edit) Why the hell am I downvoted?

If you are cycling at 10 mph in a rush hour and holding back a block worth of traffic, do you seriously not have a slightest concern that you might be valuing your own interests a bit too much? And then you wonder why-oh-why the cars drivers don't love you with all their hearts. Jeez. Do not do unto others what you don't want to have done to you.