I haven't seen the crosstabs on the USC polls, but my gut feeling is that you're right; the kind of very fine demographic reweighting they're doing isn't common, because (A) it's a pain in the ass and of dubious real-world value; and (B) it's vulnerable to magnifying sample-size and selection problems, of which the skewed Trump support amongst young African-Americans is one example. So while I'll be interested to do a retrospective on the poll's accuracy and precision after the election, I'm personally not putting much faith in it for now.