Air travel is a somewhat unusual case because, unlike most other products and services, the rights and responsibilities of each party are spelled out by a contract that goes into effect with each purchase. I think the following example is more germane to the situation than any "gallons of milk" analogy:

You need a cell phone for only 6 months. Verizon offers pay-as-you-go for $100 a month, or 1-year contract for $50/month and $200 early termination fee. You buy the year contract, use it for 6 months, then cancel and pay the fee, total cost $500. Had you purchased the PAYGO plan for six months, it would have cost $600. Did you steal $100 from Verizon, or commit "theft of services" by paying for a subscription and taking a more valuable, shorter plan?

No, of course you didn't. You broke a contract and Verizon took the recourse that they were allowed by the contract (the $200 penalty), agreed upon beforehand by both parties. This is what makes it a fundamentally civil, not criminal, matter.

When a customer breaks the hidden-city ticketing rule, the airline is similarly granted recourse options by the contract: if the customer skips the first leg, they forfeit the entire ticket. If it's the last leg, the airline may charge the customer the fare difference between the ticket they bought and the ticket they flew. As in the phone example, these penalties are in place to make the wronged party "whole" in the event of a breach of contract (if they choose to do so). The failure of airlines to exercise their legal rights in most cases of hidden-city ticketing doesn't mean that the customer has committed a crime.