They're implying that the far-right protests caused the "Arab ghetto kids" in question to become the "angry underclass", not that the "angry underclass" was itself far-right. The sentence could definitely be written more clearly.

I think it is a wrong thing to imply. Just looking at the crime statistics for the past decade debunks this hypothesis.

The arab demographic has always been troublesome in Scandinavia.

I interpret it as "individuals from an angry underclass" vs "far-right groups [that] have organized bitter protests against immigration"

So the far-right is implied to have something to do with these crimes, in the sense that they created an environment that triggers the crime.

The NYT clearly blames the crime on.... the people committing the crime. The NYT is not blaming the crime on the far right. If anything, they are blaming it on the Arab ghetto kids which is what the OP was claiming was the truth.

They are suggesting the far-right is angering the underclass. If you're asserting the NY Times is therefore blaming the crimes on the far-right, good luck!