I haven't seen anything that would question it's legality. It's a storage container that happens to have an aero design, and it complies with current USAT rules. Here is the USAT rule that would apply:
(e
) There must be no protective shield, fairing, or other device on any part of the bicycle
(including frame, wheels, handlebars, chain wheel, and accessories
) which has the effect of reducing resistance. Aerodynamic carriers for food, water, and or cycling provisions may be attached to or be an integral part of the aero-handlebars if they meet the following guidelines:
(i) The carrier can pass from front to rear, through a rectangular loop with dimensions of 10 inches (25 cm) in width and 8 inches (20 cm) in height. (ii) The front of the carrier is behind the cyclist’s hands when the hands are placed in their customary position on the end of the aerobars, and (iii) the length of the carrier is no greater than 10 inches (25 cm) front to rear.
When the carrier is an integral part of the aerobar, this integrated unit must be able to pass through the rectangular loop defined in (i) above and the carrier portion of the unit must meet criteria (ii) and (iii). In the event that compliance with this section is in doubt with respect to any particular carrier or integrated unit, a member of USA Triathlon may submit the carrier or integrated unit to USA Triathlon for evaluation.