Toggles for filtering conferences with ranking

Is this an intentional design feature or a bug that one cannot select CCF-A/B/C conferences with Non-CCF (similarly for CORE and THCPL ranking) ones in the filtering? If you click on Non-CCF the selection of the CCF-A/B/C would be canceled. The code related in checkbox_button.rs:

if value_for_update == NON_RANK_VALUE {
    set.clear();                  // selecting Non wipes A/B/C
} else {
    set.remove(NON_RANK_VALUE);  // selecting A/B/C removes Non
}