Sometimes robots sign up for huge masses of these emails, so they can sign up on forums with them etc. Basically they're not reliable enough- although it isn't a problem if you have a code verification tool (and one that actually works and isn't obvious to robots which actually are beginning to get past some of them. see the CAPTCHA site). And anyone can sign up for this email in a minute or two and so it's not reliable, it might just be used and disposed of.

It's a bit paranoid really, but that's their choice.