Yes, this seems to be one of those non-sense issues with the rule-set. Anyhow, what is a big "-" for us, is that it's most probable that everyone else also has this DOOR-STEP protection to their capital. In that case, we really can't rape a neighbour with our triremes like we had planned.
Well if we block someone's access to the mainland with Triremes (should be possible with just 2) they're as good as dead anyway. Whether this is a good thing for us or only enables the next guy to get more land, that's another issue...