Because of the guaranteed attribution.
imho, the first gives fulsome opportunities to Dremel Man, thus undermining any guarantee that the item is right.
Any attribution of an unsigned piece is no more than an educated guess.
I realise you are being specific to Venini, but operating in a field such as mine, where everything is unsigned, there is little need for educated guesswork, as huge swathes are documented.
I guess the major difference between glass and ceramics is the material. A backstamp on ceramics is not normally visible from the front surface, whereas the transparency of glass may mean that the signature can actually impact on the viewer, directly. Perhaps the reason why much glass comes with removeable labels.