It’s a discount offer, I don’t think it actually elides tax payment. According to the URL you cited:
“B&H will collect and remit state sales tax in accordance with state sales tax laws and regulations. So, customers do pay required sales tax and do not need to keep track or file anything separately.“
Yes you just get credit card cash-back in the amount of your state sales tax. Which is not bad, living in CA for instance it gives you 7.25% cash back. Compare that to Amazon where you get 5% back with their card, or any general purpose credit card where you won't really find better than 2% rewards for online shopping.
“B&H will collect and remit state sales tax in accordance with state sales tax laws and regulations. So, customers do pay required sales tax and do not need to keep track or file anything separately.“