When those popups come in, people make money off of those, kind of like ads on a website, but less scrupulous. Oddly enough, people actually do buy those things. I mean, people actually click on the "Shoot the monkey, win a prize!", and other such retardedness. All it takes is a very, very small percentage of the people who got spammed for there to be a profit.
I dunno, what I find confusing is that sometimes spammers specifically target people who specifically say they do not want to be spammed (like fake do-not-spam lists).
Then there's this: I once recieved spam about a custom logo they knew I didn't want which they bugged me about every hour for almost three years (no joke). I couldn't block it because they used a different sender every time. Finally, I managed to filter it with Thunderbird and got rid of it entirely by using Xemian Evolution to create fake error messages.
Would some people in my situation really eventually cave in after three years and buy the product? They were targeting webmasters.
Then again, that could just a company that didn't know how to spam properly.