/setblock x y z end_rod spawns upside down end rod

0 is the data value for upside down rods.

But it is weird that default position is upside-down...

I wonder if this is because they may have originally intended it as a ceiling fixture and then adapted its code to accept other directions as data values.

0 means upside down, 1 for rightside up and 2, 3, 4, 5 for north south east west, for all blocks with rotation, if that rotation isn't valid, it grabs north, this has been the way since rotatable blocks where implemented