You still have to purchase the book, and once you do it is in your account anyway so why not sync?
On my Kindle, I *can* use the browser to purchase a book, have it auto-queued to go to K4PC, and then go to
http://amzn.com/manageyourkindle to download it manually. But it downloads the exact same file I would get by syncing, so why not use the builtin method of downloading from Amazon's servers?
Are you saying you use MYK via the Kindle browser to downlosd books that you buy via the Kindle browser?