I raised the issue of red rush, and I believe that my question about red rush remains unanswered.
I wasn't asking why it was banned, but why it was banned in this particular way.

Unfortunately, I am unable to answer your question. It will remain unanswered.
This is the only point from the initial rules that I haven't had time to edit. In the second edition, I simply copied it unchanged from the first. The author of our original rules, invisible, took the wording from the Portal. Why red rush is banned in this specific way and what deep meaning there is behind it, I don't know. I think there is no political subtext here, and the author of the rules didn't harbor hatred towards red since the Civil War, nor did he have a fondness for blue.

It is not yet clear what exactly prevents blue from defending cities if they are valuable to him.

What prevents him?

And if blue defends, then red can no longer attack with an increase. Why should additional difficulties be created for red?

Additional? Does red have few difficulties?
At one time, I was very interested in debates about which color is better. I analyzed statistics in tournaments. Last time, Valera GoedenDag calculated it in Favorites, but we didn't even post it on the forum. The win rate is approximately equal. A difference of 3-5%, sometimes favoring red, sometimes favoring blue.

Firstly, the goal - "to give blue the opportunity to buy out the increase" - is a dubious goal. Whether or not blue will have such an opportunity should depend on how the opponents played up to that point.
But with the current wording, where there is a special approach to cities, it turns out that blue may not defend cities at all, and red must still deal with chains.
And if he doesn't figure out the chains, he won't be able to, for example, take an empty intermediate city with his last hero in the chain and settle there with an increase. And blue will strike back precisely with an increase.

Blue fundamentally lacks the ability to buy out red's increase during the first day of the week.
Everything else depends on the players themselves. I completely agree; whether blue has plans to buy out something, and red has plans to build chains, this is a matter of game strategy between the opponents. If blue believes that giving away his city without troops would be very advantageous, then, of course, he can choose not to defend cities; it's his decision.
In general, I already wrote that this is a question of tactics. The ban/permission of red rush is just a different game, nothing more. You need to clearly know how the rule is formulated and think about how to benefit from it.