In fact, I can write down any return address I want. Now when Jossi sends his real, legitimate tax-form to the Belastingdienst they will throw it away without opening it thinking that it is part of an attack. This is exactly what I want them to do. You got the analogy by now. Attackers on the Internet attack a Web site by sending it so much traffic that it can't handle anymore.
Attackers can make the traffic look very normal and they can put the wrong return address in it.
Remember that all these tax-forms go to the Belastingdienst by mail. I can't simply go to the Belastingdienst and drop hundreds of tax-forms on their desk. I will be caught and arrested. I have to be somewhat anonymous. I drop all my nonsense tax-forms in a mailbox close to where I live.