The point is liability, gun manufactures and dealers can be liable for the products used in killings by third parties...even after lawful sales.
Take the case of the Sandy Hook victims that sued Remington. Initially their case was dismissed, because the lower court rules the manufacture is shielded from any liability under Protection of Lawful Commerce in Arms Act (PLCAA), but on appeal the Court overturned the ruling and declared in fact the victims families could sue under State law on separate causes of actions/theories. In that case they were suing Remington for violating the States Fair Trade Practices Act (on the factual basis that Remington marketed military style weapons to civilians). So you could just as easily shrug that of and say "well, liability there sprang from...", of course liability has to spring from somewhere.
So in the case of tech, if you wanted to sue the platform, you need an underlying cause of action...whether that may be defamation, or trademark infringement, or copyright infringement. Liability for the tech platforms would have to spring from somewhere just like any other cause of action.
