If I have not mistaken, I recall Dr. Stephen Tong once said he'll never understand this part. Since it was Dr. Tong, I try not to dismiss this verse likely.
Now, through Redeemer's West Side Men weekly devotional email, I've stumbled on the same issue. We are studying the book of Judges, and we use Tim Keller's book: Judges for You.
Keller, referenced Michael Wilcock:
The force of 14:4 is that the two communities are so interlocked that even the Lord can find nothing to get hold of to pry them apart. He uses Samson's weaknesses, therefore, to bring about the relationship with this irresistible girl from which so much ill-feeling will flow. ~ The Message of Judges, pg. 139
Keller: God not only fulfils His promise in spite of their sin, but even through their sin.
Thus far, this explains quite well. Today, this kind of society union is with the liberals, post-modernism. Worshipping peace, not God. But nothing is new, it happened before, in Samson's time, where Israel befriended the Philistines.
Verses of note:
James 4:4 "friendship with the world is hatred toward God".
Acts 2:23 "How God accomplish Christ's salvation for us through wicked choices of humans"I think Joseph's betrayal by his brothers could be included in this list as well (Genesis 50:20).
New Pharaoh forgetting the credit of Joseph.
Of course, this certainly answers whether God has done this elsewhere, but has the real question been answered? Why did God choose this way?
It would appear that Keller is taking this view:
The more "sleeping/rebelling" sinners God delivers, the more wickedness of the people (including those of His own people) God shall use to accomplish his promise/will.
This may be a good answer to Dr. Tong's question.