Why doesn't the polar side of the plasma membrane block nonpolar diffusion?

Polar and nonpolar molecules don't actually "repel", it's that polar molecules attract each other much more than nonpolar molecules attract anything. Therefore, to take a polar molecule from the (polar) water on one side of the membrane and bury it in the nonpolar region in the middle of the membrane is difficult because it means breaking its relatively strong interactions with the polar water molecules without forming new strong interactions to compensate.

In the case of a nonpolar molecule passing through the membrane, it is already in water when it approaches the membrane, and transferring it past the polar head groups isn't much "worse" (there's likely a slight effect of the often charged head groups attracting ions that bridge them to one another, but this would be small). The large effect of it being nonpolar, however, is to make it less unfavorable to move it into the middle layer of the membrane where there aren't polar groups for it to interact with.