well, now you know! :-)
but when you hover over items before clicking to choose them, the tooltips also states "needs road conenction" if it is soemthign that needs it.
basically any actual hosue like building that uses workers, like hosues themselves, woodmill, barrel place, and such need road connection.Famr doesnt need any tenting, so no worker and therefor road connection needed.and fields are seen by the game the same as berry bushses and such. objects that they can jsut walk to over the terrain, no road needed.
otherwise, you would kind of have to encircle your grop fields with a circle of road pieces.or, as you did, only half fill them.