Thesis: any server push technology inevitably breeds spam
Consider various server push technologies, where things come to you instead of you having to seek them out: email, instant messaging, voice over IP phone server, and even text messaging on cell phones. All of them have spam problems (generally growing).
This is not a coincidence. Any server push technology will get overrun by spammers, because server push inherently gives them access to people and is thus very, very attractive. As a consumer of server push technology, your only recourse from the onslaught is to hide, to block, to filter; you can't actually get away.
(The push technology provider can't keep all the spammers out, if only because sooner or later some of them are in its own marketing department.)
Client pull technology is much more resilient. The spammers have to be attractive to get you to visit even once, then genuinely interesting to keep you around, and you can easily get away. Thus it is a feature, not a problem, that things like syndication feeds do not have a server push option.
(And indeed much of the spammer activity in client pull technology like the web is about being attractive, for example getting a high Google search rank for some valuable keywords.)