I can see where this appears counterintuitive, so here's why disabling IP addresses is a paid for feature.

On our end, tracking respondents is simple; it takes more time and resources to ensure that responses are completed anonymous.  Tracking IP address adds value to the majority of our users who like to see where their responses are coming from.  Also, many people use SurveyGizmo for more than just surveys.  For those using our service to create short polls and quizzes, having such tracking information is useful.  It's easy for us and good for the majority of our users, so it's a free feature.

The need for anonymity is more common among our specialized users such as researchers, who need to have anonymity.  It is more difficult for us to disable this information and ensure that no identifying information is unwittingly passed over to the survey creator, so it is why people pay to be blind.