These sort of promos have been around on the internet for a long long time, back in the day when i was a pretty hardcore gambler we took advantage of a ton of these offers. Basically you look for the ones with the best terms then play a simple low variance game(or if you're like me go for the gusto).
basically you might win or lose but over the long run you've got the advantage when the bonuses are thrown in so you deposit do the playthrough and withdrawl. go to the next casino rinse and repeat. Basically the opposite of how the casino traditionally wins your money.
Back 10 years ago there were literally hundreds of different online casinos with extremely lucrative bonuses and playthroughs, now though the industry has consolodated and playthrough requirements are significantly more restricitive decreasing your EV but there are strategies to maximize it.
Without going into a long and boring post, for those who might be interested in taking advantage of these offers you can find more info here
http://tinyurl.com/2b8nzcn
I have no affiliation with the aforlinked site. tiny url'd because of it's name.