It was actually free money assuming you had the resolve to stop once you claimed the bonus (presumably most people didnt). As long as you were laying the other side of your bet on an exchange you could extract the full value of your free bets risk free.

I collected a few grand back then, however those sort of promotions are now illegal where I live.

There are sometimes free lunches out there to be had. They’re playing the averages and accepting some losses if they know they’ll make it up from other customers. But you’d better be sure you’re not one of the suckers if you do it.