That's one way to do it (the best way). You can still draw "causal" conclusion on observational studies (where you don't get to explicitly assign people to group A and B, but where those assignment are given to you), but it is much harder and you have to be very careful with what you are doing. If you are interested in that topic I suggest you google "Rubin Causal Framework", or check out some of the papers by Donald Rubin (arguably the pioneer of the field of Causal Inference).