A discount has the psychological message of get the awesome deal before it goes away, and most people aren't aware of how that psychological message impacts their subconscious. So they tend to buy things that normally they wouldn't have, simply because it's a good deal and it has to be done now. So the cash grab comment refers to that Target is likely banking on the TMs to spend money on stuff they wouldn't normally pay for, and that 10% off is going to end up being x% more than normal.
For it to be a good deal, someone would have to carefully plan what they normally would be getting, get only that, and completely ignore all thoughts of "Hey, normally that item is $20 but today it's an extra two dollars off. It'll be useful and I better grab it because I don't know when there's going to be two dollars off again."