Partly, they believe they're dispensing something useful. (Yes, really).
It's like the confidence of missionaries and evangelists who spread like cancer through the world, sure that they have the one true message and that they're doing "good" to the "less fortunate", even when they can't deal adequately with their own lives. (I'll leave contemporary examples in the Middle East to your imagination). I always find it cheering to read of missionaries hacked to death by the people they're so keen to "improve"!
Anyone who subscribes to the Microchip technical newsletter "microSolutions" was treated this month to the CEO's 2-page article on "Personality-Driven Culture versus Consciously Designed Culture" and an advert for his book. None of us signed up for this self-congratulating sewage. We just wanted to learn techniques for using the company's products. How many of us concerned with technical matters are even in a position to exploit this guy's "pearls of wisdom"?
But they send it anyway. As you say, it generates ill-will. I'd like to read Microchip's technical information, but I won't do that if it's used as a vehicle for irrelevant stuff. I'll give them another chance, but it will take only 30s to add them to our spam list.