Wrong. If your question is whether "they" (the carrier) could charge you for the one-way, you haven't read the t&c as well as the terms of the debit issued for your original ticket (paper or online charge).

A carrier which found this to have ocurred would simply refare the ticket and issue a debit for the correct fare difference. If you read through, you will see that you are obligated to pay the difference. You would have the option, here in the USA, and in most countries of initiating a chargeback, but would lose. This is no different than a hotel's recourse if it turns out that you swiped the towels and you've already checked out.

If the question is whether this is likely to happen if you do it once, the answer is that it's very unlikely.

The more likely result in general, and sometimes the greater hook is that the carrier simply zeroes out and closes the FF account. For people who bank 500K+ miles, that is a devastating hit.