The following assumes that the beamsplitter cube or plate is made of isotropic material like glass, and that the refractive index of both halves of the cube is the same. Birefringence and polarization can complicate the story.
So: imagine removing the arrows that are inside the beamsplitter (they just confuse the picture) but leave the arrows on the transmitted and reflected beams.
Now imagine flipping the diagram on the left first on a vertical axis, then on a horizontal axis.
That flipping process does not change anything about the physics of the situation, but it makes the flipped diagram identical to the diagram on the right. Now you can easily see the % of light transmitted and reflected.
A plate beamsplitter (one face antireflection coated, the other face thinly aluminized) will work essentially the same way: the transmitted-to-reflected beam ratio will be the same regardless of whether the beamsplitter is used in the forward or backward mode.