The only legitimate topic for a treaty is what occurs BETWEEN nations, not domestic and purely internal matters.
Why did the Founding States limit Congress's powers with Section 8 of Article I if the Founders were also okay if the Senate and Oval Office used their constitutonal authority to negotiote treaties to expand Congress's powers beyond Section 8? It doesn't make sence to me. But more importantly, it didn't make sense to Thomas Jefferson either.
Remembering that Jefferson was 2nd vice president of the US, and therefore president of the Senate, here is what he wrote about common-sense limits on the power of the Senate to negotiate treaties. Note that one of the excerpts below is actually from the Parliamentary Manual that Jefferson had later written for the Senate.
"In giving to the President and Senate a power to make treaties, the Constitution meant only to authorize them to carry into effect, by way of treaty, any powers they might constitutionally exercise." --Thomas Jefferson: The Anas, 1793.What Article V of the Constitution requires the Senate to do when it negotiates a treaty and recognizes that it doesn't have the Section 8 authority to negotiate an aspect of the treaty is the following. The Senate is supposted to work with the HoR to propose an amendment to the Constitution to the states which, if the states choose to ratify the amendment, will grant Congress the specific new powers that it needs in order for the Senate to negotiote the treaty.
"Surely the President and Senate cannot do by treaty what the whole government is interdicted from doing in any way." --Thomas Jefferson: Parliamentary Manual, 1812.
Sadly, the Supreme Court compromised Jefferson's common-sense insight to Section 8 limits on the Senate's power to negotiate treaties in 1920. Note that there is no reference to Jefferson's writings concerning treaties in Missouri v. Holland where majority justices sided with Congress's ploy to force the states to comply with bird protection laws negotiated in a treaty with Canada.