Ever wondered why there is no spam in your Gmail inbox? It is because Big Google hires spammers to send specially designed spam that can be easily be resisted by GMail spam filters. They have prior knowledge of what kind of spam to expect.
Other web mail providers do not have such privileged information and will end up having less efficient spam filters and more spam.
Gradually customers will tend move from web mail services with more spam to services with less spam. And Google gets more customers to click on their ads. Profit!
Not so evil huh?
(I made up this shit. So if you choose to believe; May Occam’s Razor smite you down!)