My understanding is this is considered a "rebate." I view the cash reward as recovering part of the merchant fee charged for the use of my card. It's not income because it's my own money. Now, when I use the card for business items that I'm reimbursed from my employer 100%, in theory those rebates should be taxed, in practice, there's no tracking.
I am in the US. My current card gives me 2% back, with no limit. This goes into a 529 account (A college savings account) which as of early-2017 passed $32,000 in value.
I had the pleasure of taking advantage of a 10% reward card for a brief time, mentioned in an article on my blog, with a further link to the full story. I add this only to note that even the 10% cash back, and I never saw a 1099 for this. This year, I applied for, and got, an airline card, 60K miles for the first $1000 I charged. I suspect that's a case where I should expect the tax bill.