This is more a social norm than a rule. The forum has nothing against multiple accounts in a campaign, it's indifferent at best to the whole system. This rule is in place by individual campaign managers, and the companies that hire them. The latter is why the rule would stay in place. They are paying an individual for their service - advertise this signature with that account for whatever period of time.

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.

Now enrolling in separate campaigns can be likened more to holding several different jobs. There is no issue there, you can offer to only advertise for 1 campaign for a larger payrate(unlikely) or continue to manage multiple accounts in separate campaigns.