Can you post a link to the "US Building Code" you're referring to? I can't find it and I have a hunch that it doesn't exist.
I believe this should do it:
https://law.resource.org/pub/us/code/bsc.ca.gov/gov.ca.bsc.2010.02.1.html#p3982010 California Building Code, section 1126A
I don't think any US building codes
require the use of any particular material, just require that whichever material used is used in such a way that the Code is satisfied, ie. that your structure is "safe". That California code does not require that a standard door frame be reinforced to withstand an earthquake (more than the rest of the structure), so most door frames are build to hold doors, and nothing else.
The old-style adobe houses where the door frames are the only things still standing after an earthquake, are probably not up to code, at least the modern building codes (I assume). This article from earthquakecountry.info speaks about that:
http://earthquakecountry.info/dropcoverholdon/RE: fires and building with wood: Modern American buildings are not generally built to last a very long time. Cost is a much larger factor in buying a home than is durability. So as long as a house is up to code, most people are satisfied with their dwellings.
The building codes are designed to ensure a minimum quality of safety. "Safety", in this case means just that you'll probably escape the structure alive. Houses are built like cars here: They do their job, and if there's an accident, the priority is on protecting the occupant at the cost of the thing.