Faxon spends money to obtain original manuals and have them unbound and scanned. Then they have to print and bind a copy for sale. Therefore they include the watermark in order to have you buy the copied manual. They have printed original manuals for sale as well as some of them in digital form. It is a small business and should be compensated for their expenses.
They were kind enough to give me a tour of their warehouse when I drove there to pick up original shop manuals for my cars. They are good people.