For example, most Hackerone customers exclude denial-of-service issues because they don't want people to encourage to bring down their services with various kinds of flooding attacks. That doesn't mean that the same Hackerone customers (or their customers) wouldn't care about a single HTTP request bring down service for everyone for a couple of minutes. Email authentication issues are similar, I think: obviously on-path attacks against unencrypted email have to be out of scope, but if things are badly implemented that off-path attacks somehow work, too, then that really has to be fixed.
Of course, what you really shouldn't do as a Hackerone customer is using it as a complete replacement for your incoming security contact point. There are always going to be scope issues like that, or people unable to use Hackerone at all.
I am 100% certain that every one of the companies that paid the researched would consider the way this was handled by that researched "the best alternative to HackerOne rules 'ethical disclosure' in the face of a vendor trying to cover up serious flaws".
In an ideal world, in my opinion HackerOne should publicly revoke Zendesk's account for abusing the rules and rejecting obviously valid bug payouts.