When a field is hidden by CB Conditional its value is reset. So the checkbox is reset to unchecked. There is no option to specify a default value for the reset (there are no plans at this time to implement this) and there's no option in CB it self to specify a default other than for new registrations (plans to expand this to existing users sometime in the future).
You can hook into the jQuery of CB Conditional and extend it with additional behavior using CB Auto Actions and its Code action on the onBeforeUserProfileEditDisplay trigger if you're familiar with jQuery. The cbcondition.match.true event in your case would be used and is bound to the element the condition is configured on. Example as follows.
Code:
$( '#cbfr_FIELD_ID_HERE' ).on( 'cbcondition.match.true', function( e, condition ) {
// CODE HERE
});
There's also cbcondition.match.false if you need to match against the condition not matching.