It is an absolute PITA to keep an email server on the “nice” list so your company’s email traffic doesn’t get spam filtered by every service provider, and the major services (gmail, outlook, etc) are all federating their spam filter lists so many times if you get blocked on one you get blocked on all. There is so much spam to deal with that the filtering is highly automated and there’s little human oversight.

The point being, it could only take a handful of incidents reporting a company’s email as spam to ruin their reputation and result in email from their domain getting automatically filtered everywhere. So, you know, if they don’t support an easy way to unsubscribe then they are in fact behaving like spammers, so flag them and let them deal with having their domain blacklisted.