If the casuistry is far too extensive to the degree not every angle and permutation can be covered (and it would be), one would expect common sense to at least apply where common sense is due.
For example, if a world famous personality earned money to sit in a press conference because they have been hired to promote tourism to the Seychelles and wears a "I love the Seychelles. You should visit soon" t-shirt, you would expect them to use their common sense when they are giving answers.
Operators can run their campaigns however they want as long as they're within the rules of the forum, but I think things like this should be stated upfront as part of the terms of the campaign if it's an issue but I don't think users should be expected to refrain from promoting competitors without warning. I can see from the operators perspective why they wouldn't want this just like if you were advertising for Pepsi they wouldn't want you advertising for Coca Cola or promoting any other drinks but this would all be laid out in their contract and I think it should be the same here and made part of their rules if it's something they don't want.
Really? I think it's simpler than that, it's understood when you join a campaign. If we get into specifying everything as it is done in the contracts we will not cover all the possible casuistry even if the OP occupies two pages. Contracts are full of specifications and even then they sometimes end up in court, which is not going to happen here.