Having just read the book I've "bought" on Kindle, I find I don't actually own it. I have, without having had it drawn to my attention, bought a licence to read it. So, I can't pass it on or sell it. Now, I didn't have to pay postage on it, and I'd have to pay postage I'd have to pay to pass it on. Still, a point worth considering. If I downloaded a book and then sold hundreds of copies of it for a silly price, but one that still showed me a profit, the whole copyright system would collapse. Sony Reader has a similar system.
One of the best uses of Kindle is to read copyright free material, eg from project Gutenberg, which has all of the classics and is, of course, free.