For what purpose?
What is this naive question?
This question is intended as a question for you. I do not know your thoughts in their entirety and this is merely an act of communication: no ulterior motive. I just want to make sure I understand your whole argument rather than create an accidental strawman.
Are you going to fight those who attack users that enrol several accounts in campaigns actively?
Sure: after all, as of the time writing this post, I still see no logical precursor for this rule.
If there are no such users, keep the rule.
I don't follow. The situation you are creating is one where the rule is implemented either directly via a genuine rule or indirectly via retaliation from other users.
Like I said, this is trivial; I could fill probably half of all decent BTC running campaign without blinking. Is such a case a desired outcome? At least with a rule, there's a deterrent in the form of punishment (you'd get kicked out and likely neg. rated afterwards ruining your accounts).
It is equally trivial to prevent your accounts from being linked. Again, I fail to see the practicality of the rule -- I might understand the rationale once you indicate the negatives of an individual's account list encompassing a significant portion of the signature campaign.
If you choose to enroll multiple accounts in one campaign you are essentially ripping off your employer. Most campaigns have a minimum post count to qualify for payment, that doesn't mean that is the full scope of work for the week. That is just the bare minimum. Reaching this doesn't mean the company is getting fair value for their advertising dollars if you were to say load in account 2 then 3 for a total of 75 posts being paid 3X. When in reality they should have received the 75 posts under 1 account for 1X payment.
This is under the assumption that users exceed the maximum. You might get a few extra posts from those that are borderline spammers (take a look at some of the lower-end campaigns) but most of the time you will see people just post up to the cap and stop.
Moreover, if they wanted more posts, they could simply ask for that post requirement. Am I wrong?