Because both Hilariousandco & actmyname were left negative trust on hua_hui account, who got caught enrolled his alt in his own campaign (i've told this above). Which is same with pawanjain, who enrolled his alt as well in campaign he managed.
To make it clear the forum has no rule that we can't enroll our alt in the campaign we manage. The case you are talking about has the rule in the campaign that alt accounts are not allowed.
If the basis for rule-following is
just obeying the forum rules, then the default state of the forum would be nothing more than a bunch of polite scammers that refuse to copy each other's scam techniques.
I would probably say that the act of using your own clandestine alt isn't the most trustworthy way to do things, especially if you could simply manage the campaign and advertise it in your signature at the same time. The incentive of being paid according to one's post quality in a limited campaign makes it easy for a manager to judge their posts in a biased manner - where one might be less inclined to spam in other campaigns, someone who both runs and participates in a campaign could effectively reward themselves for spam to maximize their time. This is especially important if they had multiple accounts and had to create a constant slew of posts.