NOTE: A Lot of this post is from memory I've done my best to make sure it's coherent and not rambling...
Eru does not intervene because everything has its purpose even evil.
Because without evil there is no good.
For example, apples to oranges if you'll allow me, in "The Invention of Lying" the world doesn't seem better off because of a lack of lying(something most consider wrong). Alternatively it's impossible trully to consider one side evil and another good. Consider a Goblin for a moment would you dub them evil for being against the dwarves? Possibly, we are afterall following their tale in The Hobbit. But from the Goblin's view the dwarves are trespassers and consider the wielder of Glamdring a villain.
While it can and usually is argued Melkor's desire for power is evil or at least very wrong, it could also be countered that it's simply ambition. After all we don't consider Manwe as desiring power yet when challenged by Melkor he defends his crown, surely if he had no desire to rule he would have surrendered the crown.
Also we consider Melkor's destroying the trees as eviul yet without it the balance of the world's night and day do not exist. We considered it evil but it was neccessary.
If you remove sin from the equation(as it may be argued a mere perception) you'd have to consider sickness and drought and winter, perhaps even Death itself as evil. But without these neccesities overpopulation and mass famine would occur which perhaps is even worse.