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Using Google's reCAPTCHA as an incentive to purchase services or products without the explicit consent of Google Inc. not only circumvents the intended use of the API (to stop spam), it directly contravenes Google's Terms of Service, and leaves the website operator liable to...
1. Jurisdictional Income Tax & Labour Laws
2. Fees & penalties related to, or in connection with; misrepresentation of the intellectual property of Google Inc., misappropriation of income(s) accrued by the aforementioned violation of User Agreement.
3. Criminal penalties arising from the use of software and/or services which allow a visitor to bypass or circumvent the CAPTCHA for a fee.
In brief, if this website intends to charge for a service, or the removal of a synthetic hindrance (in this case, a CAPTCHA), it may not use Google reCAPTCHA or any Google Inc. API without prior written consent.
The next time you post a link to the imageboards, please quote the relevant information.
Also, check 'em.