It's a massive issue. It's not only spam, it also enables malware distribution and e-mail bombs (flood of mail to cause DoS or to hide some other letters). It

It's a massive issue. It's not only spam, it also enables malware distribution and e-mail bombs (flood of mail to cause DoS or to hide some other letters). It is mostly because proper defenses against such abuse aren't built into software allowing such forms (or cost money). Wordpress form plugins are one such widespread bad example.