My Doctor told me that you do not have a real period on the pill because you do not ovulate. Therefore the bleeding that occurs every month happens because you are not taking an active hormonal pill, so you shed the lining as you would without the pill. Essentially you are going through hormone withdrawl and bleed because of it.
That is why pills like Seasonale can work, you just take active pills the 4 week for 3 months and then you can have a period every 4th month. Since it isn't a real period, you don't need to bleed every month, but you must have a period at least every 3 months.
If pregnancy should occur while on birth control you would miss your period entirely becuase the fetus would need the lining.
I'm trying to remember everything my Doctor told me on this subject, when I was inquiring about why Seasonale would work, but I'm not sure if this is all? Maybe someone else can shead more light?