That's how you get gravity in a space station -
rotation.
(There's little point in mounting the mail-slot on a giant wheel and counter-rotating it so that it appears to not be rotating relative to the planet, because once through the slot you'd just need to start rotating the ship anyway)
Luckily FDev have gone easy on us and made the ships automatically start following the spin once inside, but you can turn off this feature if you want to experience attempting to dock without computer assist
(Edit: the shape of the mailslot comes from the original Elite game, which in turn got it from the movie version of 2001. So the shape is Because Tradition

)
Here's that famous scene:
https://www.youtube.com/watch?v=q3oHmVhviO8
This is also why the Docking Computer module plays Blue Danube Waltz
And I assume that the Kubrick chose the waltz as the perfect accompaniment because the docking is a dance that requires each party to be rotating through space in perfect alignment...
(Ooh - and probably also why the grill mesh projecting out from the mailslot is shaped the way it is - check out the docking computer screen in the ship in the clip!)