Likely for the same reason that someone like fyrstikken gives money to a select good steemers or kly reformed his online presence. To gain more reputation and influence.

See discussion over here https://steemit.com/trolls/@stellabelle/steemit-s-secret-weapon-turning-trolls-into-functioning-members-of-society#@the-ego-is-you/re-stellabelle-steemit-s-secret-weapon-turning-trolls-into-functioning-members-of-society-20170102t203221109z