Why is court permission needed when Islam permits polygyny?

Court permission does not forbid what is lawful; it is a governmental administrative measure to ensure that the Qur'an's condition of fairness is truly met. Allah states that if you fear you cannot be fair, then only one (an-Nisa' 4:3). The court's role is to filter out applications that would clearly lead to injustice, such as a husband unable to support his existing family. Scholars accept that the ruler may set regulations for the public good (maslahah), as long as they do not forbid what is lawful, and the people are asked to obey those in authority (ulil amri) in what is right (an-Nisa' 4:59). Registration also protects the rights of wives and children. See polygyny in Malaysia.