Cash back offers are common with things ranging from TV's to Car, to lots of things in between.
I've only bought one item with a cash back offer. It was on a 55 inch TV from TCL, cash back offer of $70. I wasn't expecting it, but thought I'd redeem it.
The payment did not come from the vendor. I was [eventually] paid by TCL Australia.
TCL made me jump through hoops, including providing a lot of information from the box in which the TV came. This went on for several months. Finally they said the offer had expire.
So, I went to the ACCC ( Australian Competition and Consumer Commission. They have teeth, and can (and have) issued million dollar fines.
The ACCC said such an offer can only expire if that is clearly stated from the beginning. EG on the box.
Once I explained that to TCL, they paid the $70 the next day. I didn't care about the money, but I cared about being cheated.
Overall, it seems to me that the consumer needs to be very stubborn to redeem most of such offers. I gather it's relatively small amounts. Doing some research, I discovered that around 70% of consumers don't redeem such offers. Seems to me that outcome is intentional with at least some vendors.