Theory: HRT were pretty late building the car, and probably have the lowest budget of all the teams, so since the ruling seems to be unclear everywhere, they decided to play it ultra-safe so they don't have to lose money and time redesigning it.
Edit: I'll explain better.
What I mean is, when they designed the car, they weren't sure if angling the exhaust downwards would be safe or not. So they decided to angle it upwards, which would be 100% safe and wouldn't force them to spend extra money on a new exhaust system. When you're on a really tight budget, the risk/benefit/cost ratios probably means you go for the safest designs.
Alternatively, when all the other teams started developing downwards exhausts, they didn't have money/time to develop them again, so they stuck with their old "upwards" design.
Either way, it's probably a budget thing. Bigger teams usually can take more risks since it doesn't cost them too much to have 3 or 4 designs already planned.