I have a hard time interpreting what the sign could have been intended to signify, even if mounted at a proper angle. Both directions are OK, so why can you only go right?

I don’t see what’s confusing about this sign, or interesting about this photo. From the leftmost of the two lanes indicated, you can turn left or right. From the rightmost lane you can only turn right. The “101 Detour” indicator is on the upper right part of the sign, meaning you should turn right if you want to take the detour.