The SDWA covers water systems, not undeveloped open water sources, so looking to it for a rationale behind warnings to the public regarding the safety of drinking directly from lakes and rivers is going to be an empty pursuit. This is not a "big gummint overreach" to paraphrase certain of my inlaws.

The 1986 reauthorization did widen the SDWA's reach to include small and seasonal water systems not originally covered and has certainly led to the closure of a lot of small water systems. But that's a separate and unrelated topic.