The Merit system is intended to be used if you find a post, or the person posting it, constructive and beneficial to the forum. I do agree that this is very general, however whether it is abuse or not does - I think - depend on the people involved.

Providing that the merit system is used as intended, and how the signature campaign was run, that could be a completely legitimate way to use Merit.

For example, if a user is running a signature campaign and would only accept users over a certain threshold of merit then it would be absolutely fine. Since given merit is intended to be used as a way to gauge post quality, only accepting users with a high amount of merit would be a fairly decent way to weed out shitposters.
However, if a user was running a signature campaign and required each participant to give him merit to get paid then it would be wrong and the user would likely be warned/punished for such.

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This should be fun.