If you check out the American Diabetic Assoc., their studies have found that fake sugar makes people gain weight. And they used to push the fake stuff all the time.
These sweeteners were first developed for diabetics. The Claim that artificial sweeteners cause weight gain must be made by people who don't understand the complex chemistry of satiation. They suggest that the taste function can control or influence the amount of calories that go into our bodies. This is not how the body is set up and contradicts what we've learned about hunger and satiation. These people seem to believe that the body's genetic structure can be overridden.
These kinds of claims may be good for obtaining grant money but they don't make a lot of sense to people that believe research should be concerned with finding the truth rather than funding.