I sold a Brioni shirt like that awhile back. The buyer I think wore it to an event, if I'm reading between the lines correctly, and tried to return it... ended up giving him a partial refund (and a block from further bids, of course).
It IS somewhat commonish for people who don't know better / don't like it to cut the tail off of that - but yes, it is in fact meant to be fastened just like a onesie. There's a button on the lower front to use for this I think. The idea is that way, you won't have to worry about your tux shirt coming loose and untucked as you dance / move / whatever - after all, when wearing a tux it can be a major pain to readjust like that, let alone "appropriate" if you're at the kind of event that you'd be wearing a tux to.
Too bad the sleeves aren't 36" or I'd be snapping that up - I found a Borrelli tux shirt on ebay but it... isn't quite as traditional as I thought when I bought it, though it'll do.