1. Calvinists often say God "permits" evil. Some Arminians say it's misleading or meaningless to speak of divinely determinate events as divinely permitted events.
2. To begin with, the usage varies with the Calvinist. Paul Helm uses permissive language. He's defined and defended what he means by that. Here's one example:
Scroll down to the "willing permission" section.
Calvin himself was ambivalent about permissive language. Calvin made the elementary observation that you can't drive a wedge between what God wills and what God permits. Divine permission is either willing or unwilling. If unwilling, it would be coercive rather than permissive. But if permission is willing, then God wills to permit evil. Yet in that event, what's the big difference between willing evil and permitting evil? If he wills to permit evil, then he wills evil. To will to permit it is to will it. The circumlocution doesn't eliminate divine volition in the matter. At best, permission indicates God's grudging attitude towards the relative necessity of evil.
And notice that this applies to freewill theism, not just Calvinism.
3. Let's compare two questions:
i) Why does God permit evil?
ii) Why doesn't God prevent evil?
These are equivalent questions. They convey the same idea. The only difference is that the first formulation is positive while the second formulation is negative.
4. Moreover, this is consistent with predestination since God could prevent evil by not foreordaining evil. Therefore, it's not contradictory for a Calvinist to say God permits evil.
5. I myself have no particular attachment to permissive language. However, in discussing the problem of evil, I often frame the question in terms of why God permits evil.
An Arminian like Jerry Walls, who assumes the worst about the Calvinist motives, might suspect that I use permissive language to conceal the true nature of Calvinism. If I were more forthcoming, I'd come clean and phrase the question, "Why does God predestine evil?" The fact that I avoid that either means I'm lowballing Calvinism or that I'm conflicted.
But as I just demonstrated, that language is consonant with Calvinism.
Moreover, I've often defended the claim that God predestines evil. I'm not running away from that fact.
6. I generally use permissive language for two other reasons:
i) It's the stereotypical way in which the problem of evil is framed. And since that's consistent with Reformed theology, there's no overriding reason to depart from that formulation.
ii) But more importantly, I don't usually phrase the question "Why does God predestine evil" because that has the wrong emphasis.
That formulation suggests the question at issue isn't so much about God and evil, but about predestination. Why does God predestine evil, in contrast to evil coming about some other way.
But although that's worth discussing in its own right, the problem of evil centers on the divine rationale for the existence of evil in God's universe. Given that God could prevent evil, why doesn't he? What possible reason could he have not to prevent it?
That's why I generally use permissive language in framing the issue. To phrase the question in terms of predestination would distract attention away from that central concern.
Moreover, once we discuss the purpose that evil serves in God's world, that can naturally segue into a discussion of predestination. But doing that in reverse is less logical.