For the same reason if you add a crocodile logo onto a polo-shirt, it still doesn't make iti Lacoste.
Sticking a badge on something without the brands authorisation makes it a fake.
Newbery, whilst they don't make their own bats (and haven't for about 10 years), still design and commission the manufacture of cricket bats. They also spend decent sums on their brand, whether that's the marketing or ensuring the integrity of their products. Whilst most of us don't like the outsourcing to India approach, they do have a right to ensure that their logo (which has signficant value and would therefore add value to a plaint cricket bat) is only sold on products they're happy with.
Which is what makes selling authentic stickers on their website such a strange, and risky decision! Not that I didn't buy a few sets!