Right, because insinuating anyone who uses mibbit is incapable of installing an IRC client is not a sniping remark?
No, simply that as an initial barrier to entry for a technical community, "browsing to a web page" isn't very difficult and serves poorly as a first-pass filter.
webmail clients are only blocked 'due to abuse' because it's easy to make bots to register fake email addresses. But even that is not fool-proof. I remember that back when AOL on dial-up was my internet connection, the 'master' account could create multiple 'child' accounts. So even the ISP in that case could allow you to create a 'fake' email for the purpose of registration.
No, I regularly find modern registration forms that won't accept mailinator or even hotmail addresses (and your service is probably more akin to mailinator than hotmail).
No, simply that as an initial barrier to entry for a technical community, "browsing to a web page" isn't very difficult and serves poorly as a first-pass filter.