In my own opinion Campaign managers asking users to comment a certain number of characters is both good and bad and probably to avoid spamming. And I think why campaign managers add at least 150 to 200 characters is simply to avoid one line spam posts like: “I agree” or “great project” e.t.c . I believe that the forum should have quality discussion and not all this empty post to get paid.
I wonder how the forum would be if such a mindless approach is allowed. The CMs are even doing the forum a favour by making that clause mandatory, and I wonder how a barely one forum line post + a few extra characters should be an issue.
Besides, those companies paying for campaigns here are not doing it to waste their time, but for marketing. The lesser the inputs, the lesser the possibility of the SEO. Even though they do not want to burden anyone, yet semantic load is still important for what they paid for to be actualised.
Above all, it's a choice to accept the campaign, and if you do, you must be capable. The 150-200 characters should be a walkover for any serious person.