Credit card companies will give you free money (a portion of what they charge merchants), and that really has nothing much to do with how to travel cheaply.
If someone said "here's how to get $500 of stuff on amazon for $50" and the trick was to use $450 in amazon rewards earned from normal credit card use, that'd be a dumb article, right?
Yes but you're ignoring that $4000 spend on Amazon Visa might net you a $40 certificate (@1%, I dunno the exact reward) where as $4000 spend on British Airways Chase might net you 75,000 miles (incorporating their bonus) which might represent $1000 worth of airline ticket.
In my experience, assuming you fly, the airline reward cards are the best in terms of the $ value of the reward.
edit: Maybe some airlines do it by only letting you use reward miles on seats that they wouldn't have been able to sell anyway or something along those lines. But some don't. I know Southwest reward tickets aren't heavily handicapped or anything lame.
2) $1000 in airline fare isn't actually a $1000 liability to the airline, because that is the market price where as to them the cost price is going to be less... and as you suggest if they restrict it to a few seats per plane the cost price is even less.
And yes, reward flights usually require you to pay tax, and for popular routes there may be only one FFM redeemable ticket per class per flight. Meaning you have to book ages in advance, choose less popular routes/times or be damn damn lucky.