Hi,
this is not a feature, but you can don to this with javascript like this:
jQuery(document).ready(function(){
jQuery('.jsn_registration_controls button').attr('disabled','disabled');
jQuery('#jform_terms_of_service_alias0').change(function(){
if(jQuery(this).is(':checked')) jQuery('.jsn_registration_controls button').removeAttr('disabled');
else jQuery('.jsn_registration_controls button').attr('disabled','disabled');
});
});
replace terms_of_service_alias with alias of field like #jform_ALIAS0 instead #jform_terms_of_service_alias0