I think it’s because the captcha code may change after it’s validation.
It may remain the same for a few times, but it will change at last when you continue trying to be validated. So the author of Yii-user wants the captcha to be validated only one time at the last moment of the registration.
Sorry, the description above was not correct.
The validation of the captcha is performed in every ajax call.
By default, the captcha will remain the same for 3 times and will be changed for the 4th time.
So the author doesn’t want the captcha to be changed in ajax actions.