Opting not to tip when it is part of the economic transaction is no different from walking out with the silverware; not expressly forbidden, just a breach of social contract.
And walking out with silverware is theft, I genuinely have no idea where you pulled that from as a similar example.
There's lots of evidence that tips vary significantly based on the traits of the customer (like the customer's self-esteem and sense of shame: https://www.emerald.com/insight/content/doi/10.1108/ijchm-02...) and the employee asking for the tip (e.g. attractiveness and simple demographic characteristics: https://www.sciencedirect.com/science/article/abs/pii/S01674...).
Isn't property theft very expressly forbidden?
Well, shit, if I made it part of the economic transaction, you'd have a point. What you're saying is that the employers are not holding up their end of the transaction.