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Not all registrations are done by bots. there are also 'datacenters' where humans are payed to do this, with some automated assistance.
In some cases the captcha's are presented on other websites as 'allowance', without verification, but thus retrieving the solution for the bot.
There are some catcha hacks (recaptcha, sexy captcha) as alternative to the regular XOOPS captcha, search for captcha in the forum.