Hi @wadcom, welcome to our forum! You’re asking a great question. And honestly, you have as much right as I do to come up with your own answer. As @hender99 said, there are no rules in music. The only role of music theory is to try to describe and organize what human beings are already doing. And in the blues, the most common scale played over that 4D chord would include b7. So this raises the question that you are asking: why?
My own attempt to explain this (which I stated in both the IFR book and the IFR Blues Mastery Course) is that in the blues, we never hear the 1 chord that comes from the major scale. In other words, we never hear the first harmonic environment like we study in Seven Worlds. Right from the start, the tonic chord appears as 1D. So this establishes the scale 1, 2, 3, 4, 5, 6, b7 as the basic harmonic environment of the song.
Then when the 4D chord comes along, you’re right that it doesn’t require us to play b7. But by the same reasoning, it also doesn’t require us to play a natural 7. So if we’ve already been hearing the b7 from the 1D chord, why should that change when the 4D chord comes along?
As I also stated in my book, it doesn’t really matter very much whether you find this explanation convincing. It doesn’t amount to much more than mythology. Trying to explain why the b7 sounds so natural to our ears in that moment is like trying to explain how the zebra got its stripes. One could imagine all sorts of imaginative explanations, but we don’t actually need any of them.
We already have everything we need with the mere presentation of a couple of different groups of sounds. One group of sounds is the 4D harmonic environment that we find in most jazz standards, which indeed has the natural 7 that you were expecting:
4, 5, 6, 7, 1, 2, b3
Another group of sounds is the 4D harmonic environment that we find in the blues, which has the note b7 as follows:
4, 5, 6, b7, 1, 2, b3
I think that the impulse to try to explain the origin of common music practices is a healthy one. It’s natural to try to find the logic or meaning contained in these choices. In music, that mostly takes the form of simply noticing when one set of sounds reminds us of something else. Whenever you find a set of notes that is surprising to you, there is almost always a very simple logic explaining the relationships between those notes, and this logic usually comes from some other chord or scale from another key which we are merely “copying” and superimposing here.
But as improvisers, it’s not really our “job” to have to come up with any convincing explanation for WHY certain sounds are played more than others. Our job is to get to know ALL of the sounds, and then create our own culture by expressing the sounds that WE want to express. So I feel that the best thing you could do with your current fascination for this topic is to take this opportunity to experiment with both the b7 and the natural 7 over the 4D chord, and just notice for yourself what effect each sound has on the music.
Remember also that the chord column drawings that you see in the IFR learning materials were never intended to limit your melodic choices to those notes. All of that harmonic analysis is merely our attempt to give you a roadmap of the sounds that we think you will find to be most consonant in each moment of a song. In other words, the IFR chord columns are like a roadmap of the underlying harmony of a song. But they do not in any way limit your freedom as an improviser to paint whatever sound you like over that canvas.
In my own life, I don’t know if I can even remember the last time that I improvised over a blues without playing both the b7 and the natural 7 over the 4D chord at some point. As both improvisers and composers, we always have all 12 notes of the chromatic scale at our disposal at all times. And once you’ve gotten to know all of these sounds, it’s hard to resist playing all of them. 
David