Create failed: creating a new record is not allowed for the caller.

I'm trying to create a very simple signup form using the API "create" client-side method. Based on the documentation for the create method, no authentication is required ("The methods available from a web client do not require authentication of the caller."

However, when I send all the appropraite information, I get the error response "Create failed: creating a new record is not allowed for the caller."

If creating a record doesn't require caller authentication.......... how can the caller not be allowed to create a new record?

Here's the HTML, like I said, very very simple (I plan to make it a whole lot more complex, but if the API doesn't work as indicated I'll need to start over and I'd rather do that now than later:

<form id="cons_signup_form" name="cons_signup_form" action="https://my.care.org/site/CRConsAPI" method="post">

<input type="hidden" name="add_interest_ids" id="add_interest_ids" value="1222" />

<input type="hidden" name="api_key" id="api_key" value="-OMITTED-" />

<input type="hidden" name="v" id="v" value="1.0" />

<input type="hidden" name="response_format" id="response_format" value="json" />

<input type="hidden" name="method" id="method" value="create" />

</form>