Spam and the attraction of reach
Here is a thesis: the larger or more standardized the environment for sending messages, the more spam you should expect to get in or through it. Accordingly, email is heavily abused because it is hugely standardized.
The spammer's motivation for abusing larger, standardized environments is obvious; the larger the environment, the more people you can reach with a single technique, approach, or system. Larger environments have better return on effort, since generally (but not always) most of the effort in spamming in an environment is figuring out how to do it well.
(This ties in to how spammers are lazy but not stupid, at least not in the aggregate.)
This is depressing because it implies that any well used service that allows push messages is going to have spam no matter what you do. If you build such a service or protocol and it gets popular, you'll get spam. (In fact, degree of spam is not a bad metric for degree of popularity. And if the spammers abandon you, well, worry.)
It is tempting to say that one important way to discourage spammers is to shift the relative costs so that as much effort as possible is per-message effort; if nothing else, this might make you less attractive than the next target. However, I think that the general history of people's anti-spam efforts in new systems shows that this ultimately doesn't work; if you're attractive enough for regular users, you're easy enough for spammers.
(See also DeterringAbuseProblem on this general issue.)