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.
It’s moral theft. Accepting service from someone and refusing to pay for it in a situation where established convention is to pay for the service. If the service was poor, then it’s not theft. But if you appreciate the service and still refuse to pay, you are morally a thief, regardless of what the law says.