Why would they be blocking it? Google promotes 8.8.8.8 as the primary dns server, and, as far as I know, as long as it keeps answering, clients won't fall back

Why would they be blocking it? Google promotes 8.8.8.8 as the primary dns server, and, as far as I know, as long as it keeps answering, clients won't fall back to the secondary 8.8.4.4.