OK, why do people do this? They say things like "Can I ask you a question about something?" Typically on irc, but even in real life some times. Worse, some times people just say "Can I ask you a question?" !!!!
I expect part of it is the use of the word can. In my mind that is asking if something is possible, i.e. physically possible, doable, can happen. I.e. "Am I physically able to ask you a question..." which is a daft thing to ask - only you know if you can, or not, and generally, barring having a heart attack just then or suddenly going mute, you can indeed ask the question. After all, you have just shown the ability to ask questions by asking the first question :-)
However, even assuming the more likely meaning "Do you mind if", which makes a lot more sense, you then start to be playing in the realms of etiquette, which is a minefield in itself. If one has to have permission to ask a question surely one needs to have permission to ask the first question, i.e. I may mind being asked "Can I ask you a question?". I suppose this very rant suggest that is the case! So presumably the first question is a break in etiquette (doing something I do mind) in order to avoid breaking etiquette by asking the real question straight off... WTF?
I should probably not be trying to apply my tactless, and mostly logical, mind to any sort of social etiquette really should I? :-)