From what I can tell, modems are meant to block redstone on sides where the turtle has a peripheral (such as modems). However, visually it does not connect on the opposite side instead.

The redstone API functions work as expected: an output can be set on the right (without modem) but not left (with modem). I believe this is because TileComputer swaps the east and west directions, but TileTurtle does not. getRedstoneConnectivity should use the opposite side instead - it is passed in the direction from the wire, rather than the side of the block. It also appears that there is a null check missing (the docs say side can be null).

Tested on Minecraft 1.8.9 and ComputerCraft 1.79 without any other mods.