I do not think they are considered retro. Interventional studies are widely considered the "golden" standard but they are not perfect and, in many cases, not feasible. Like you point out, in many cases, it would not be ethical to perform one ; for example, making people smoke on purpose and then try out smoking cessation interventions on them. "Luckily" though, people usually self-select their selves into these categories.

Judea Pearl disagrees.

Links about Judea Pearl:

http://www.danielpearl.org/about_us/Judea_Pearl.html

http://ftp.cs.ucla.edu/pub/stat_ser/R320.pdf

The usual link to Peter Norvig (director of research at Google) on experimental research, which is good food for thought:

http://norvig.com/experiment-design.html

He disagrees with the idea that traumatising children is retro, or with the causation vs causality? It's been a few years since I read the book, but I had his do() operator in mind when I wrote the comment.

Edit: Okay - I clearly have to clarify my comment. Intervention studies are not retro. Studies in which you deliberately traumatise children, however, went out with Ceacescu. I was making a joke (albeit apparently a poor one).