You're ignoring a lot of other factors here.
The actual sequence of events was:
- Melkor stomps around the world being EEEEEVIL.
- Melkor is chained the first time and imprisoned in Mandos.
- Melkor is released and does more EEEEEVIL.
- Melkor is chained the second time and is CAST OUT FROM THE FRICKIN' WORLD!
- Saruman does some bad stuff too.
- Saruman is denied return to the Blessed Realm.
So, the OP is written as though the events numbered (3) and (4) here did not exist. Melkor's treatment the first time round was lenient for the following reason (quoted from the Silmarillion):
Quote:
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For Manwë was free from evil and could not comprehend it, and he knew that in the beginning, in the thought of Ilúvatar, Melkor had been even as he; and he saw not to the depths of Melkor’s heart, and did not perceive that all love had departed from him for ever.
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The
second time round, however, the Valar were
not taking any of his sh1t. They'd already made their mistake in being lenient and they'd already seen the consequences of that. So it's off with his feet and out into the void for the remainder of eternity for him.
By that yardstick Saruman's fate wasn't unfair; he actually got off fairly light.