Why 'email marketing' winds up as spam
The problem with email marketing, why in practice it turns into spam, is what I will call 'address list creep'.
The cleanest list of addresses to send your marketing email to is people who specifically requested that they receive them. But even at the best of times, this is not quite the same as 'people who have specifically subscribed themselves to a mailing list that you run', and this difference creates a strong pressure to let people in your organization add addresses to the mailing list.
This might work in theory, but in practice marketing and sales are populated by people who are at least optimists (and sometimes worse). So your careful list of people who requested email broadens to include people who wanted it without specifically requesting it, and then broadens to include people who the marketing department are sure want it, and then broadens even further. And then you are spamming people, because even with the best of intentions your organization went one optimist too far.
(For all of the obvious reasons, the situation gets worse if your marketing and sales people have quotas or other incentives to generate 'leads' and do not have the quality of those leads checked rigorously.)
A similar thing happens to addresses that you gather in these carefully maintained mailing lists. Sooner or later someone in marketing is going to win an argument about how, if people on your list signed up to hear about A, they clearly also want to hear about B. And then you are spamming people again, and in a way that is especially damaging (because you have not merely spammed people, you have betrayed their trust that you would not spam them).
(None of this is original to me, but I feel like writing it down.)