Keep in mind that the mass media is driven by ratings/viewership/readership. This overrides any other motives they or their corporate owners may have. Running polls and announcing them is a huge ratings-getter as the election approaches.
Also keep in mind that they aren't "news" organizations any more... they deliver infotainment. They need short, sharp, exciting tidbits to throw at their customers, and polls are a great product of this type. But to keep the attention of their audience, the polls have to be both reassuring and anxiety-provoking at the same time. So pumping out polls showing Hillary in the lead by only a small margin is a great way of achieving that.
Voter manipulation is a secondary, albeit real, motivation.
