This looks like it is caused by the client side validation. If you go to the page and click on the SPRY checkbox element and then in the properties inspector, change from Required (single) to Enforce Range (multiple), the for should submit even if the checkbox is not checked.
Note: In the code, you will notice that changing from Required to Enforce Range will modify a line of code similar to this one:
var fieldset_group_field_Spry = new Spry.Widget.ValidationCheckbox("fieldset_group_field_Spry", {validateOn:["blur"]});
To look like this:
var fieldset_group_field_Spry = new Spry.Widget.ValidationCheckbox("fieldset_group_field_Spry", {validateOn:["blur"], isRequired:false});