Roam belongs to the semantic class of verbs which share the feature of movement in a non-direct path.
Direction is the most salient parameter of these verbs, as they all necessarily involve non-linear motion. Their direction is ultimately determined by the non-linear nature of the verbs or constraints placed on the path itself.
The path followed during roaming is determined in the first way by nature of the verb, one must roam in a non-direct path. The path itself plays only a secondary, non-binding role.