What PSU specifically are you using?
My PSUs all have exhaust fans. Typically exhausting upward from the fan out the top of the PSU. I would agree an intake fan would appear backwards.
I have one of each. I just tried the second one, to compare airflow direction.
Intake/forced air does seem weird, but I'll figure it out.
The APW3+ suck are in on the power plug side and exhaust on the power connector side. So yeah, for most installations they vent hot air in the direction of the S9 intake fan. Not a great design.
Thanks for clearing that up.