I understand but again it's only 20PPM
20 PPM of what? That's only a measure of a given contaminant, or does it mean that there is only a total of 20PPM of all the possible contaminants that are in treated mains water in rainwater? Where in the world is that and where does that information come from as I don't believe that any credible organisation would just state ' there is only 20PPM of all possible contaminants in rainwater compared to tap water' that's a too wide and sweeping statement that couldn't really be substantiated. Heavy industry isn't really the issue now, amongst other things then cars/vans/truck/buses/factory pollutants/dust and dirt from farming picked up by the wind, even radioactive material can found in rainwater and a lot of these pollutants are naturally filtered out when rainwater filters through to become ground water.
All the information I seem to see these days is that there isn't any rainwater anywhere on the planet that is safe to drink any more without some type of treatment.
Lets also not forget that normal drinking water tends to contain minerals it has picked up whilst ground filtering that are quite essential for our bodies, these are not present in rainwater, so they would need to be supplemented. That and the naturally alkaline PH of groundwater, is know to be good for the human system, slightly acidic rainwater isn't and again would require treatment. I am not arguing that our treated water doesn't have chemicals that have been picked up/added that may be of concern to some but suggesting that raw rainwater is far cleaner is, IMO, quite a dangerous assumption.
I am not saying that rainwater shouldn't be drunk but it would not be a good idea to have it straight out the barrel, all water we drink these days needs treatments of one kind or another. Let's forget most of the water that is drunk these days, no matter how it comes out of the tap, has been rainwater at one time or another.
Oh and taking our wonderful Scottish Water as an example, there are very few of the chemicals you have listed in our water. That's purely environmental and we don't add fluoride either so our tap water can actually be better for us than bottled mineral water, especially if left in a container in the fridge to allow the chlorine to evaporate.