Well, I'm not not going to declare that it's a bug or not a bug. That argument has been debated here often in the past as well. I don't make a habit of attacking via ports unless a faction emerges way out ahead of the others (and me) due to circumstances like a Daimyo dying and the majority of that clan's assets switch loyalty to another, already powerful faction, etc. If they developed as a result of me overindulging in development without balancing that with steady and prudent expansion, then I don't use port raids. In the 1580 Tokugawa campaign for MI, the cyan clan begins with only three provinces, but a nice little contingent of Battlefield Ninja, so I do use those to port raid from the get go (they are only 108 men). This allows the Tokugawa to survive without having to take ridiculous risks until the other clans have slugged it out a bit and the board balances out. Sometimes the ninja end up having to fight or be eliminated against 5-10 times their number because they are cutoff, so I think it's "fair," but that's just me. I think it's up to the player. In SP, how the game is played doesn't affect anyone but the player, so my take is, play the way you enjoy.

From a programming standpoint, being able to attack via port seems like deliberate intent, as it wouldn't be possible without someone actually programming the cability. But I don't know--we'd have to ask one of the developers.