Look. The aspect of “theft” comes into play by the IRS. The government.
When the IRS takes tax revenue from an income YOU DID NOT make, THAT is theft.
The restaurant customer maight be a total jerk, or maybe the service was terrible... who knows?
But the failure to leave a tip is not theft. It might be mean, cruel, justified, whatever...
But unless you show me ONE case of a non-tipper being arrested....
It’s not theft.