First - not all airlines are created equally. American started this promo and they allow you to fly any fare to earn the free ticket.

Second - I don't know the details of the Delta promo, but if the rest of the Terms&Conds are anything like AAs then your example is not correct. You picked one route and according to your theory the free ticket will be used on the same route. AA promo will give you a free ticket to anywhere AA flies. So I could fly 2 trips between TPA-LGA which I could get as low as $156 R/T, so after spending $312 total, I will get a free ticket that will get me to Tokyo, Buenos Aires, Rio, London, Paris or wherever I choose.

Third - I agree with some others here. Even looking at Delta's exemption of the lowest fare, you're assuming that everybody flies on lowest fares. They don't. Just because a promo may not work for you, it does work for many others. I'm mostly leisure flyer so I have to watch the fares. But many people do fly on higher fares because they need flexibility, refundability, etc.. AA has another promo that if you fly 10K miles in Jan. any miles above that will be triple miles, BUT the miles after the 10K can't be on the 2 lowest fares. The promo is great, but for me personally it won't work, yet I don't see it as a fraud. The airlines are choosing to reward the higher spending pax, and I can't blame them.