There’s some value in the public suffix list being shared, with mild sanity checking before accepting entries: it maintains a distinction between site (which includes all subdomains) and origin (which doesn’t). Safe Browsing wants to block sites, but if you can designate your domain a public suffix without oversight, you can bypass that so that it will only manage to block your subdomains individually (until they adjust their heuristics to something much more complicated and less reliable than what we have now).