Why is a person who dies as an unbeliever (kafir) punished by staying in Hell eternally? Is it a just punishment?

One of the explanations of Badiuzzaman Said Nursi related to the issue of why a person who dies as an unbeliever needs to stay in Hell eternally is as follows:

"Question: The sin of unbelief persists a short time but its punishment is eternal and unending; how is this conformable with divine justice? And if one does accept this, how is it consistent with pre-eternal wisdom? And if one accepts this even, how does dominical compassion permit it?

Answer: If one accepts that the penalty is infinite, it is established that unbelief perpetrated in finite time is, in six respects, a crime of infinite proportions:

Firstly: The person who dies an unbeliever will remain as such even if he lives to all eternity, for he has corrupted the very substance of his spirit. And his corrupted heart has the propensity to commit infinite crimes. Therefore, his punishment is not contrary to justice.   

Secondly: Even if unbelief occurs in finite time, it is an infinite crime and gives the lie to infinity; it denies the whole universe, which testifies to divine unity.

Thirdly: Since unbelief is ingratitude for infinite bounties, it is an infinite murder.

Fourthly: Unbelief is a crime against infinity; that is, the divine essence and attributes.

Fifthly: The human conscience is, in regard to its outer face, limited and finite, but by virtue of its reality, the roots of its inner face spread and extend to eternity. In this respect, therefore, it is infinite. Unbelief, however, sullies it and it dwindles away.

Sixthly: Although opposites stubbornly resist each other, they are similar in many instances. Thus, on the one hand, belief yields the fruits of the delights of Paradise, and on the other, unbelief produces everlasting suffering and pain. It may be concluded therefore if one puts these six aspects together that infinite punishment fits the infinite crime and is pure justice." (see Nursi, İşaratü'l-İcaz)