What tier-1 provider is irresponsible enough to use this?

Spamhaus is huge. A listed IP will see email bounce rates exceeding 50%, easily. Most large North American ISPs use at least one Spamhaus list. Tier-1s don’t generally advertise which blocklists they use, but the 2010-ish “Stophaus” DDoS attacks went after a number of Tier-1 operators.