Courts are not inconsistent, they are following the law. Like most cases decided by SCOTUS, they are deciding on the rules set by Congress and courts have ruled time and time again, that Constitution gives Congress almost sole jurisdiction for determining how Copyright law functions.

For copyright law, Congress does not expressly allow secondary liability for third parties FOR COPYRIGHT unless the party induced the infringement or the provided service is tailored to that infringement. In this case, Cox was not cutting off copyright infringers BUT since their service could be used by same infringers for valid use, they didn't have to.

For arms producer, Congress has exempted them for liability and courts have ruled, yep, Congress gets to make the rules here.

Congress could overturn both rulings by changing the law.

This is what we want. Congress makes the rules, courts interpret but don't make new rules.