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Would you reward a Newbie with his 1st merit point, if you see a decent post of his and/or engagement in discussion, but his posting history is, in vast majority, bounty-campaign proof posts? Do you consider such people as spammers, or you just ignore campaign-report posts? After all, they don't pollute the forum and are contained in dedicated threads.
As long as you think that the post deserves merit and you have some, why not?
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Because it shows that primary reason for their activity is for-profit posting. Therefore rewarding them with merit (effectively ranking them up and enabling their signatures) increases risk of them jumping straight into signature-campaigns to shit-post one liners all over the forum in very high volumes.
That's why.