I REALLY don't get this. Clinton wrote a law I can't carry in, or near a public school. FEDERAL law.
Wow, you are really totally out of touch with reality on this issue. That 1990 law was overturned by the US Supreme Court way back in 1995 ( United States vs. Lopez ). The new law that the Congress passed to replace it exempts people who are licensed by the state to carry concealed weapons. So if you have a concealed carry weapon permit, you can indeed legally carry on school grounds.
Your information on this subject is literally over 13 years out of date, and totally incorrect.
Now just knowing the correct Federal law does not get one off the hook here, as you need to know your own state's laws about carrying guns on school grounds also. Some states don't have any restriction on CCW permit holders carrying at schools, while others do. But that is a totally state by state issue.
WHY do people put up with private business telling them they can't carry?
Well, you don't have to if you want. Since they make this a requirement of doing business with them, you can simply boycott their business. Employers can do the exact same thing, and require that employees not bring guns into the workplace.
You can also violate the rule, and risk the consequences. Since you are not breaking the law, you are not guilty of any crime, and thus do not risk arrest. But in the case of an employer, you would both lose your job, and also be ineligible for unemployment benefits. In the case of a shopping mall, you could get expelled, maybe even banned from the mall, if people notice that you are carrying and become concerned and report you.
Schools can have rules that ban firearms themselves, and many of them do. For example, at many universities, if a student brings a gun on campus, they will get expelled, losing all of the time and money they invested into their college education. This applies even if the person has a concealed carry permit, and is thus legally allowed to posses on campus.
This became an issue here where I live when the local university had a rash of rapes several years ago. Female students that had CCW permits asked the school if they could be allowed to carry on campus, because of the rapes. However, the university denied their request, and kept their 100% ban in place.
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