If you have a field you want to grow valuable crops on, but the soil is too poor, a way to fix this is to spend the first few years growing a less valuable but hardier crop. Eventually, when the soil is good enough, you take your plough to the field and till the old plants into the soil, where they act as fertilizer for the new crop you then sow.
The obvious simile: this is what happens to users and small content producers once a platform matures. It's what's happening to tumblr right now, with Yahoo marking blogs as NSFW based on opaque criteria and reportedly making tags such as "gay" unsearchable.
It's what happened when Microsoft dropped XNA support: when the XBOX was a new platform in dire need of games, Indie developers were a welcome sight. Now, they're an unprofitable distraction.
It's what happens every time when "more stringent submission guidelines" are instituted, or large companies start getting special deals, or discoverability of content is exchanged with curation and sponsored links. It's the platform operator saying "Thank you for creating this vibrant community. We will now till you into our field to make it fertile for our business partners."