I'm with you on that, although it's something of a double-edged sword; for many people it is decidedly Not OK, for instance, for an Anglo-American writer (to use your example) to "borrow" Native American figures and tropes for their fiction.

I think that the melding of different so-called Old World mythologies and legends can be interesting in an American context--American Gods if, say, American Gods had been good. But you've struck on the problem exactly, in that part of the problem of coming from such a young culture, or a young blend of older cultures, is that the sort of deep resonance you want for your mythic work is much harder to reach.