The reasoning was that it was easier to implement that way. CTEs can contain state changing behavior and for that reason they are treated as optimization fences

The reasoning was that it was easier to implement that way. CTEs can contain state changing behavior and for that reason they are treated as optimization fences in order to be as safe as possible. I believe Tom Lane has since said that there isn't any reason it should stay that way and the community has never received any guarantee that this was going to stay the same. It just hasn't been implemented. In other words, they're seeking contributions. I'd do it myself if I were even a remotely capable C programmer, but I'm not.