You are wise to wonder about the catch. Of course there is a catch!
Long story short, ALL of these sites were created for ONE REASON ONLY - that is to TAKE your money, not to give you any.
Even if you win with that "free bet", you won't be able to take any money out. First of all, there is a long, long page of "Terms of Agreement" that you'll have to click "I Agree" just to sign up for the free bet. Part of that agreement is that you need to "play through" the money you receive about 30 times. That will mean hundreds, and perhaps thousands of dollars worth of wagers you'll need to make before you can withdraw. Second of all, you will almost certainly be required to make some sort of deposit with real money before you're ever allowed to make any sort of withdrawal.
It's all just a ploy to fool you into whipping out your wallet. Once you do that, they've got your money, and you'll never get it back.
Truly - only a fool would believe they will get something for nothing from these sites. It seems you've got a good head on your shoulders. Keep it that way. Don't get fooled by anyone spamming you with any crap that tells you otherwise. These people are vultures just waiting to get a piece of you.