It's quite reasonable, you are "hired" as a campaign participant as "indirectly/directly promoter" of that service/product, but you instead suggest the product of its competitors when you can also mention that campaign in a list of suggestions.
It's like an unwritten rule that a "promoter" should do in a forum that avoid advertisements in their posts...
So the question is, should the forum introduce any rules on compelled/restricted posting, or do we let anything go, including shilling campaigns?
There is already forum rule about that, let me quote it with bold part.
Advertisements (including signatures within the post area) in posts aren't allowed unless the post is in a thread you started and is really substantial and useful.[9][e]
Shilling campaign is different for a single post/reply that engage discussion, answers a question and suggest a signature campaign-related product, its reasonable for a "campaign participant". Unless the poster blatantly spamming that product in off-topic way of posting.