The Helmetless Alpha Section, was supposed to be an Alpha Section-unit.
When the Lighthouse shelter was attacked, Double H warned Jade for possible left-over Alpha Sections in the shelter. As we know, there are not any troops left, but according to the picture --at one point in the game's development-- they were. It seems this particular event has been through multiple iterations. In one version of this event, the remnant Alpha Section units were supposed to wear their masks, but this iteration was also removed and thus in the final version of the game, there are no Alpha Sections encountered.
A possible explanation states that they were removed to enhance the emotional context of the scene; action would have been out-of-place on this occasion. Another explanation states that the it was merely used for marketing purposes in order to promote the product.