Fraud - No
Unethical - No
Illegal - No

They make you an offer, clearly stating the T&C's and you can choose to accept it.

If you know you need 3 tix and the price is less than the 2 in a higher fare class, go ahead an pass on their offer

If your 3rd ticket is to a very expensive destination, or you don't know your schedule but think it is worth the extra $$ to have the option of a free ticket anywhere, take the offer.

I have no problems at all. Of course, I also expect people to be smart enough to read, consider the options, and make a smart decision. I have no sympathy for the 'victim syndrome' - it's all your fault because I couldn't bother to think.