People can write a request on a Post-It and stick it to the return. It doesn't take a lot of effort.
They recognize it costs money. They see it as badly spending taxpayer money.
It saves quite a bit of money and time.
It's a practical, not statutory, decision. We're not going to spend resources to go after a dollar or two. ... If you send it in, fine. If not, we're probably not going to bother with it.