Sounds like basically they are paying for you to sign up to 99bitcoins, onc you agree to suscribe to their website they will email you a heap of junk trying to sell you something, smart company move.
$50 worth of BTC is ok, just sign up with a email address created just for this promo so as not to get a heap of junk on your everyday email address.
Actually I run the website
and I don't send out heaps of junk, just 1 blog update a week. We hold these giveaways once a month and usually there are around 3000 entries so your chances of winning are 1:1500 = %0.06. Also you can unsubscribe whenever you like, just keep in mind that if you do win we send a notice to your email so it probably shouldn't be an account you don't check.