Because Hillary had hired people to do exactly that:
http://www.latimes.com/politics/la-na-clinton-digital-trolli...
http://www.thedailybeast.com/hillary-pac-spends-dollar1-mill...
Remember someone leaked a private Slack chat of Reddit admins where one was boasting of receiving their CTR check.
If that is true, supported by the articles you referenced:
-- r/politics has over 3 million subscribers, with an average of 30 - 40 active users during the daytime. -- if every one of those users was a "shill" commenter then every single one would have earned $0.30 for their efforts. That's not counting other forms of social media.
I have to admit some skepticism toward the assertion that the entire subreddit is a controlled narrative. Instead, it seems that there will be those who adamantly support the people they're paid to support, and that those users are in the minimum. Instead I think it's just a general consensus what the popular ideals are in there vs. r/t_d. Besides, not sure if you've been around there much but I certainly don't see much love lost for HC.
That is, excepting the "shill" name-calling -- it leads me to believe that you are in fact speaking on behalf of r/the_donald. If that's true, then I know why the term was used. If not, what leads you to believe that anybody involved in discussion in r/politics are "shills".