Well...
"Even if people are around I don't indicate." - You get honked for such a thing because you're not giving anyone indication (indicator... indication) of your intentions. As such they have to guess what you're doing. People are paying attention not only to you but also the road ahead. So if you just start slowing down for apparently fuck all reason, they're going to be a little peeved when you suddenly zip off without giving them a hint first.
A honk isn't just given in anger, but to let you know that you're doing something retarded and maybe you should stop.
I admit that I slow down if someone behind me is right up my arse, but that's what you're 'supposed' to do. It's good driving practice.
"I don't know what the honker expects to happen in those situations." - Expectation is that the offender will realise they're being an arse.

Of course, the above does not apply to people acting as though they own the road.

ps - what vix0r, with a little less swearing, anger and horn pipping.
(tweebianmonkey Should be known as Widget, Mon 5 May 2008, 2:28, Reply)