It's not for spam, it's for privacy. Entering your email in a field that says "will not be published", and they generates an avatar, actually party publishes your email address, which is bad for privacy.

Say, you suspect that Alice on stack overflow bad-mouthing software vendor "HAL", claiming impartiality, is really Eve, working for competing software vendor "Moon", you might be able to confirm that by using your knowledge of moon.com e-mail addresses, and checking the hash of eve@moon.com, causing SO to breach the privacy Alice/Eve expected when signing up.