I am not a breeder, and this comment may be out of line, but if the breeder (who has the bitch with pyometra) breeds her "back to back" are you meaning the bitch will give birth to a litter of puppies, and then immediately will be bred again so she has no rest time before having another litter?
So, here's my reflection: I would think a bitch who had pyometra suffered physically as a result. So, she is bred, which again (I would think) drains her physically. So, if she is bred "back to back" (and then spayed) is the reason for the back to back breeding just to maximize the gain for the breeder (this gain could be "fame" or actual money) before the bitch is spayed?
This doesn't seem to be in the bitch's interest.
Back to back means the bitch is bred the next time she cycles. That might be six months later or more.
You're kind of making some assumptions, the least of which is that breeding is about making money, and breeding back to back just means more money in the breeders pocket, or that a bitch is so depleted after a litter that it takes at least a year or longer to recover. In either case those assumptions are not correct.
Breeding isn't always black and white. There is a lot more gray to consider in the decision making process than absolutes. A particular bitch may possess traits very important to the future of a breeding program. If the breeder loses her contribition, it can set the breeders program back several years. You have to weigh each breeding individually on its own merits.
Many Reproductive Specialists now recommend back to back breedings (assuming the bitch is healthy), over skipping cycles. This is a change from what has been the norm for decades. So, I would not automatically assume that breeding back to back is somehow bad or unethical, what really matters is the overall health and condition of the bitch when deciding to breed or not.
In the case of a bitch that has a history of pyo, you're risking the health of the bitch by leaving her intact and NOT breeding her when she cycles. If you do choose to breed, you're dealing with an "At risk" pregnancy right out of the gate. The care of a bitch under such conditions is both time consuming and expensive.