Well, it's not exactly free money, it's free travel. If you can sell it, it's free money.
If you were going to do it anyway, then yeah, by not spending the money you come out ahead.
I guess I don't really live that sort of lifestyle. If you gave me $1K in flights I'd have no use for it (which I think comes hand in hand with not paying the $5K in the first place - consumption is expensive).
If you could buy shares on a credit card I suppose it could make sense.
When I was younger, I signed up to a bunch of accounts in the UK that paid benefits. You could get 10 quid a month from Halifax, multiplied by three. Possibly six, I don't remember now.
That actually was free money. There's still stuff like that around, but being a software developer is better paid. And more interesting.