Let's talk about our Red Rush strategy...

You can position yourself on the side of the portal that red sees.

That's exactly what I'm talking about. In this scenario, blue has a choice: to block or not to block the portal for red.
But I'm talking about a situation where red can see both sides of the portal, and blue has the tactical option to cut off red's troop supply. Of course, the probability of such a situation is not very high, but anything is possible. Similarly, you can also block a narrow passage, but in that case, a separate rule would be needed for that scenario, and such a narrow passage is less common than gates/portals.