It's worse than that.
You can't justify an eternal reward for finite good either. Heaven is equally as 'unjust' as Hell, but in the recipient's favour.
Imagine an atheist and a theist die and go to Heaven, the greatest theme park imaginable with infinite rides, sublime refreshments and absolutely no queuing. Both are given guides, gifts and the obligatory halo and wings.
Only the atheist's halo is only 99% as shiny than the theist's, and her wings carry her between rides only 99% as fast as her friend.
So she now gets to spend eternity being fed, watered and entertained by sights and thrills beyond imagining, but with fractionally longer gaps between them, while fractionally less brilliantly dressed (halo fashion is a big thing in Heaven).
In comparison to the theist, our hypothetical atheist in Heaven is being infinitely punished. An eternity of minute delays and miniscule unfashionableness sums to infinite deficit. She starts off slightly less favoured than her friend and the gap only ever grows, over an eternity her reward will grow to become infinitely less than his. Even the mildest punishment times infinity is an infinite punishment, to have someone burning forever, in unimaginable torment, forever, is just a childish revenge fantasy.
Hell could be a paradise beyond comprehension, just as long as Heaven was a slightly better one.