Why is the MCLG for those set at zero?  I'd really like to see the quote from the EPA showing that they claim those levels as the "safe dose"

No, all toxicity is not the same and you can't compare the effects of acute toxicity to those of chronic or cumulative toxicity, even with the same substance let alone between different ones.  With all the thousands of gallons of water I have consumed over my life, how can I still be alive?

You are making a good argument for why we shouldn't trust the dental profession in judging the toxicity of fluoride.