The 1995 case involved a federal law forbidding anyone from carrying a gun near a school. The states all had the right to pass such laws, and most did, but the issue was whether the federal government could pass such a law under its power to regulate interstate commerce.
The underlying argument was similar to that in the 1942 case of Wickard v. Filburn: School violence can affect education, which can affect productivity, which can affect interstate commerce.
Since virtually everything affects virtually everything else, however remotely, "interstate commerce" can justify virtually any expansion of government power, by this kind of sophistry.
The principle that the legal authority to regulate X implies the authority to regulate anything that can affect X is a huge and dangerous leap of logic, in a world where all sorts of things have some effect on all sorts of other things.
As an example, take a law that liberals, conservatives and everybody else would agree is valid -- namely, that cars have to stop at red lights. Local governments certainly have the right to pass such laws and to punish those who disobey them.
No doubt people who are tired or drowsy are more likely to run through a red light than people who are rested and alert. But does that mean that local governments should have the power to order people when to go to bed and when to get up, because their tiredness can have an effect on the likelihood of their driving through a red light?
The power to regulate indirect effects is not a slippery slope. It is the disastrous loss of freedom that lies at the bottom of a slippery slope.
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