DISCUSSIONS on “Rules for online games in HoMM3 and formulas for calculating player rating.”

It's quite obvious. Doesn't the word specific clearly indicate that the rule applies to a specific – single – passage, and not to all possible passages between adjacent territories?
As for why – that's a completely different question. If there's a passage in another location, then guarding it is a perfectly logical price to pay for passage. Otherwise, there could be a mountain in that location, or simply an impassable obstacle.