can't copyright a device that has a legitimate purpose?

I read this here [www.wired.com]

"It might also be difficult to target the manufacturers of the 3D printers because of the 1984 decision in Sony Corp. of America v. Universal City Studios, Inc. The Supreme Court exempted manufacturers of recording devices from copyright infringement if those devices had a legitimate purpose."

That is copyright not patent.

This means that while a 3D printer may be used to commit a copyright infringement, as it can also be used legitimately, the manufacturer is not responsible of the infringement.

Studios tried to make all recording equipment manufacturers liable in such case which would have been serious trouble.
3D Printing is similar.

Providing services to commit this infringement is liable if they do it knowingly.

That is a very badly written article needlessly confusing. Situation is clear and actual laws cover all bases correctly.

The only thing 3D printing disturb is making regulated or proscribed parts (like firearms) that become much harder to follow. Hence the Cries of alarm we heard recently from politicians reacting like shark scenting blood in water

The Supreme Court exempted manufacturers of recording devices from copyright infringement if those devices had a legitimate purpose."

AKA you can't sue the makers of VHS recorders and DVD Burners because those devices have legal uses.

It's not the manufactures fault that people where using them to illegally copy stuff.

Same as photocopiers makers aren't going to responsible for people trying to print money.

Same as 3D Printer makers aren't going to be responsible if someone prints an illegal device.