True, but people can always post to the blockchain by other mechanisms than the Steemit web site. I don't think there's a way to require a captcha from all possible posting sources.

But now I'm wondering about some sort of captcha-based verification service. Maye a service could be set up where people could voluntarily complete a captcha periodically and then curators could use that information to guide their voting. Here's something of the kind for Ethereum.

To be honest though, as a user I'm not a big fan of captcha, so I'm not sure if I would participate in something like that or not.