seawolf,

If back to back tickets are against the airline's tariff, then it is illegal. It's called fraud--when you book the outside ticket, you represent to the airline that you will be going out Day 1 and back Day 4--when in fact, you intend to come back day 2 and go out again day 3.

The airline relies on your repesentation, and charges you only the discount fare, when, in fact, your travels mandate full fare.

All three requisite elements for fraud are there--representation, reliance and loss.

That being said, no airline is going to turn this over to the police--they will take there own remedies and be done with it.