Why is owning or using a cell phone jammer illegal?

Because it disrupts people’s ability to use a communication device. That carries a whole host of potential problems.

The only reason to use one is to be a dick.

From an FCC Enforcement Advisory, 2011…

Why are jammers prohibited? Use of jamming devices can place you or other people in danger. For instance, jammers can prevent 9–1-1 and other emergency phone calls from getting through or interfere with law enforcement communications (ambulance, fire, police, etc). In order to protect the public and ensure access to emergency and other communications services, without interference, the FCC strictly prohibits the use, marketing, manufacture, and sale of jammers.

So I guess before people had cell phones, everyone was constantly in danger.

@Zaku Yes, but there were landlines everywhere, and people weren’t jamming them. And you didn’t need to pay to dial 9–1-1 from a pay phone.

Use of jammers by restaurants is completely justified and should be legal.

Go blather on about your stock split by the valet kiosk.

At the restaurant I worked at the longest I was allowed to ignore a table with patrons on their phones. This was good policy.

Those that went there to dine better deserved my finite attention.