I have a question about nested airline tickets that someone probably can quickly give me the answer for. The question is currently only hypothetical but looks like it may become a real consideration (due to work buying some airline tickets and me buying others).

Supposing I have round-trip airline tickets (say from Fly-By-Night airlies) from a city (say Chicago) to another city (say San Francisco) that span a couple of weeks (cheaper rates for weekend stay). It's obvious that if I buy a round-trip FBN ticket over the weekend in the middle from San Francisco to Chicago, the purpose is to circumvent the original fare requirement for a Saturday night stay and FBN would (a) take all my frequent flier points, (b) charge me an enormous amount of money, and (c) leave me on the runway and grin at me as they fly by. Plus I'd never see my luggage again.

However, is it legal to buy a FBN ticket from San Francisco over the weekend in the middle to a city nowhere near the origin (such as Atlanta?) Obviously this isn't to circumvent the Saturday night stay requirement on the first ticket but is it legal? (I suspect it wouldn't be for a city near Chicago, like Milwaukee, but Atlanta is nowhere near).

I know the most common answer that people could give me is to use another airline but there may be a compelling reason (such as extremely cheap fare or desire to get the right frequent flier points.)