Torches can only be placed on the sides of solid blocks. Both farmland and grasspaths have a height of 15/16th of a full block. They are therefore considered transparent blocks since they include a transparent part (the upper 1/16 of the block). This means the behavior described in this ticket is intended.
FVbico and Connor Steppie: You are correct, my mistake. I am however interested in the point Christian Rau brought up. It would ultimately depend on what the intentions of the developers are, though...