Giving credit card details Swagbucks News/Tips

You know those offers, where you give your credit card details for the free trial, and then you cancel before its over, so you dont get billed. Are they safe to use, or should they be avoided?

I did 2, one to this hayu.com, I cancelled as soon as i registered, but have gotten no sb points. I did another now called purifiercc, it said i would get sb within 5 min. I cancelled sub but see no sb. Wtf is up with this, is this SB thing a scam? Im asking as its my 2nd day using, and alot of stuff seems very shady