But since you brought it up, cold outreach for B2B is not considered spam, as long as they comply with the following: https://www.leadfuze.com/cold-email-outreach-isnt-spam-heres...
Not everything legal is ethical, but in this case it seems more annoying than unethical. I hate cold outreach emails and my company doesn't send them, but concluding that YC doesn't care about being ethical because the former president of YC tweets about a company that sends cold outreach emails seems like a stretch.
(As my final word on it however, Zenefits specifically was not sending "cold outreach emails", it was bona-fide spam. But, they're far from alone in this anyway, which was my main point.)
I'm actually more concerned about the actual spam in my real world mailbox that USPS dumps 3 times a week, no opt-out button there.
I beat this drum occasionally because I don't want to have to pit my meager resources against the resources of someone like YCombinator who are willing to provide funding (and introductions to enormous amounts of even more funding) to companies that are OK with spamming.
And I'm not including B2B cold emails as "spam", even if they're written as a template, so long as there's an actual human behind them and they aren't being sent out en masse (for example, Locbox: https://news.ycombinator.com/item?id=4672162).
Spam is illegal, so that's one good sign it might be unethical. About 90% of email is spam [1]; the reason you aren't spending all day "deleting with the click of a button" is that a lot of smart people and a lot of computing power are devoted to keeping most of that spam out of your inbox.
You should be thankful for the people who get offended about this stuff, because its only their reactions and their hard work that have kept email a usable medium.
[1] https://www.m3aawg.org/sites/default/files/document/M3AAWG_2...
Info here - http://www.consumer.ftc.gov/articles/0262-stopping-unsolicit...
Or, might be that our definitions of unethical don't match up 100%. I consider spam to be unethical, maybe you just mean more serious offenses.