Well, I can only speak for UK Tri's as I've never done one outside the UK. Here, the helmets that we use have to meet one of the recognised safety standards either the European CE standard, the Snell standard or something similar that's recognised. The normal old style time trial fairings don't offer any kind of protection and are just for aerodynamics.
After cycling banned those, more aerodynamic safety helmets have started to appear on the market. For me, anything that can save me 30 secs on the bike means I'm that little bit closer to the front for the run...... I usually go backwards after that but there we go.
As I said, don't know the exact rules for the states but should think that they're similar.