They don't. They can always delegate to bidbots to reap the benefits of their SP without posting. That said, I'm not to any extent blaming users for shitposting.

Don't get me wrong
I'm not hating on shitposts
Nor on users who create them.

I completely agree with what you said here

The system itself encourages shit posts and for someone who holds say 500,000 SP it would be foolish for them to not post something at least once a day and upvote themselves.

But this is and will keep on changing with projects like @steem-ua, @curie other curation programs who upvote based on "quality"