I don't get it. I, too, have seen that review where the thing just popped right off with little effort. You're saying that it came off with relative ease by using it to close the tailgate or by putting groceries on the tailgate. I HAD to remove that piece to get to a screw that holds the back panel of the tailgate on when i installed a backup cam, and FOR THE LIFE OF ME couldn't get mine the hell off. The one tab that holds it in place on the side of mine actually BROKE trying to get that hunk of plastic off. I don't even have that cover on mine right now. Really should glue/epoxy that tab and put it back on. Hope i can remember where i actually put the thing. Might be in the garage..... *shrug*
But yea, i've always either used the handle that opens the tailgate to close it, or just used the top edge of the tailgate in a similar fashion to how one might use the bottom edge of the tailgate on an SUV to close that.