It's basic capitalism; if this "vulnerability" wasn't built into their code, would you submit to a more expensive price on your laptop when you could just as easily get half off (for the same product) on your phone?

If there were no 'hack', and I went on that plane knowing full well that I was going to purchase internet access, I would buy the half-off solution and tether to my phone.

As a user, this is a valid set of decisions. Since they're implementing this in a stupid way, it's perfectly valid to exploit their method and pay for the cheaper item.

If you went to the supermarket and found an item at $10, but you had the option of doing 5 jumping jacks to lower the price to $5, what would you do? Is it immoral to do jumping jacks?