Root cause vs. proximate cause. If artificial sweeteners prime the body to expect calories that it doesn't receive, and (or by some other mechanism) this causes later overeating, then it's fair to say that diet soda does cause weight gain.

As an aside, I have a hypothesis that naturally focusing on proximate vs. root causes is actually a big contributor to the divide between major segments of the population.

Control being pure water, unsweetened coffee or caffeine solution? Give me proper science.