cause it's forums , there is no harm at all when you have more than one account if you aren't abusing the campaigns and selling the accounts
some biz owners need more than one account , like one for the biz they run and one personal
even gambling sites allow more than one account if you aren't abusing the site , so it's super normal for forums to allow that too
but as I said once the rules are broken there is a good chance that all of the accounts can get a red flag , which will make them useless if you are using the accounts for campaigns or planning to sell them
May be it won't harm but definitely will make this forum full of shit and crap. I am pretty sure that out of 10 only 1 or 2 will able to handle their multiple accounts in a right way and mostly will just try to sell their accounts for money or will use it negatively e.g in scams.
There isn't any strict rule of having multiple accounts yet but it will be much needed to catch them before the forum will become overloaded with useless accounts.