The point, I believe, is that the argument that the government has no legitimate power to regulate (non-government-issued-currency
X) because in the past it chose not to regulate (non-government-issued-currency
Y) in the same manner, despite several express Constitutional grants of power that speak to relevant powers of government is, well, completely bunk.
The government does not lose enumerated powers, or the ability to apply them in a particular manner, because it chose not to apply them in that manner at one time in the past in a similar circumstance.
You can, of course, make the argument that, power aside, the government should not use its enumerated powers in the way in question and should instead act as it did in the past case, but that takes more than waving your hand in the direction of the past case and asserting that it exists and therefore the restraint the government exercised then must be repeated, it takes actually making the case that that decision would be correct in the present circumstances.