A cause of obesity is something like the lack of exercise, eating too much, constant snacking, etc. TV and Video Games are things that may cause the lack of exercise. But to say that the cause of obesity is something like a Video Game just doesn't make sense. Even if this was the case, it can't be said of every game. Look at items like Wii Fit. They might take a bit of offense at hearing that their game causes obesity.
Let's take a different scenario. Is it valid to say that the cause of you driving into a ditch was because you have a lot of friends or is it because you were texting a friend while driving? I would hope the answer is the latter or else I may be in trouble. Having a lot of friends may lead to distractions while driving (phone calls, texts, etc) which may cause you to go into the ditch. But having a lot of friends is not what caused you to go into the ditch.
Video Games and TV don't cause obesity. Playing too much or watching too much may result in a lack of exercise which may lead to obesity. If we are going to state it properly, you should say that the main cause of Obesity is the lack of exercise which is often the result of playing video games too much or watching TV too much. That statement I could agree with.
Just my $0.02.