volvie, one point you are missing, there is a very important difference between linear motion (IE in a line) and accelerated motion including rotation. Lets say you have a pickup truck and you put a bowling ball in the back, if you drive down a the street without turning or speeding up or breaking the ball will stay in one place relative to the bed of your truck. But if you should turn then the ball will feel an inertial force as it tries to maintain its momentum while the truck moves under it. This force will be equal to mv^2/r. Relativity postulates that all frames of reference are equivalent if and only if they are not accelerated.
Similarly the Earth turns around its axis once each day. As long as scales are small we don’t feel a force from that turning but in some cases it can be measured and is important. An object moving from the Earth’s equator to the north will find that it has too much angular momentum and will feel a force, exactly the same as our bowling ball in the truck above, and will start to deflect to the East. If the Earth was not rotating this would not happen.