Drinking or eating and excessive amount of anything is obviously a bad idea, as the very definition of the word "excessive" implies it is too much. But something that damages only our own health and causes no harm to those around us should be left up to our individual discretion. It is different than something like smoking, which is not only harmful to your self but also to those around you breathing it in secondhand. There is no such thing as secondhand diabetes.
Yes, we should be allowed to drink an excessive amount of soda. As adults, things like that should not be regulated for us. We are intelligent enough to know the damage that excessive amounts of soda can do to our body, and should be allowed to make our own decisions about what we choose to ingest. The government is not our mommy.