A clear, transparent, durable flexi glass may appear appropriate for all spectators and those who wish to see every action on the court, do you agree km? It gives you a better view even if you are at the back of the boards watching. Newscasters' cameramen may find these useful to cover exciting activities especially pointing to the action packed 'under the goal' or 'around the goal' offensive and defensive area.
We may better want to see our favorite NBA stars like Kobe Bryant and great dunking Griffin doing their thing on a glass board than on a dark hard board if we are watching from behind the boards, or would we not care?
It's plexiglass so fans sitting on the ends of the court can see through it. Otherwise, it would block a significant portion of the action. Plexiglass does shatter if enough force is applied, but the broken shards are not nearly as sharp as broken glass, and they are lighter and less likely to cause injury when falling from 10+ feet. Even the plexiglass boards around a hockey rink are sometimes shattered by a hard check.