Not strictly true; I remember that, back in 1965 or so, a few local racers would deliberately break off that brass nut and the portion of the valve inner core that extended above the outer valve body. Doing so guaranteed that they'd be able to inflate their sew-up tires with a Silca frame pump without risking inadvertently nudging the brass nut sideways and thus releasing all the air that they had just laboriously pumped into the tire.

As Silca pump users know, that instant rush of pressurized air out of the valve and into the pump would result in the pump handle blowing out of the pump body and flying across the street. Hilarious sight, if you weren't the one using the pump.

The only downside to snapping off the brass nut and top portion of the inner core was that the tire would have to be kept at or above a certain minimum pressure or else the inner core would disappear irretrievably into the tube. Training sew-ups with butyl tubes would hold their pressure for a pretty long time, but racing tires with latex tubes would have to be pumped up at least every few days, or else.