Copying my reply to this same point from elsewhere:
Those phones were presumably glued together and not as repairable as the Fairphone is. Which is very useful, but does lower your waterproof rating, hence the need to compensate elsewhere.
I really feel like people are too quick to assume malice, generally. Often, there are just trade-offs with no clearly-right answer, and it’s not obvious to folks like us on the outside what those trade-offs are.
What I’m assuming happens is that no way is fool-proof, so they managed to limit the amount going into the USB port, and they’d be able to limit the amount going into the jack, but not by enough to tip it over the edge of the rating.
Copying my reply to this same point from elsewhere:
Galaxy S5 not only had a headphone jack and was water resistant but it had a removable battery as well.
Could you also replace the screen, camera’s, USB port, loudspeaker and earpiece with nothing but a screwdriver?
I don’t see how is exclusion of headphone jack going to help with water resistance if everything is held with screws anyway.
My guess would be that it’s one less hole that water/dust can get in?
If they figured out a way to keep it out of usb c hole then it isn’t a rocket science to keep the water from going in through headphone jack.
What I’m assuming happens is that no way is fool-proof, so they managed to limit the amount going into the USB port, and they’d be able to limit the amount going into the jack, but not by enough to tip it over the edge of the rating.