I think there's a big difference between credit card sign up bonuses, which are basically all "above board" and travel hacking, which is finding out loopholes or ways to maximize rewards in ways companies didn't intend.

There's nothing wrong with advertising credit card sign up bonuses, since businesses want to get customers to sign up with these terms.

The kind of middle gray area is advertising things like the citi AA 75k cards or the AMEX "bump the bonus" tricks. It was never really clear whether or not these offers were completely sanctioned by their parent companies. Or at least it seems plausible that the company leadership might have been unawares as to the scale of what was being given away to savvy customers.

Now it seems the flood of people bumping bonuses on AMEX cards has caused AMEX to stop giving out courtesy bumps entirely. A classic example of how the Internet changes the ball game. Twenty years ago a few people would have thought of the idea and called in to talk to a CSR asking for a favor and gotten it, and maybe some of their friends and family would have repeated the trick and gotten in on the deal. Now though, a few forum and blog posts and suddenly AMEX is $10 million in the whole in terms of rewards points.

Whether or not this is a good thing is hard to judge, but it seems to be a fact that deals once publicized get shut down pretty quickly. (Although the Citi AA cards are still going....)