Regarding how Pierce hurt his ankle during the extended off season: Doesn't matter. Life happens. he could have been working out like a dog all off season, been in the best shape of his life and had someone hit his heel with a shopping cart at the supermarket or he brought his leg down too fast, hitting his heal, while stretching on the floor or something. Accidents happen.
Actually surprised we haven't seen more injuries around the league given players weren't allowed near their training staffs, their team training facilities or their team doctors for that amount of time.
Regarding teams reporting injuries: In contact sports it makes sense to me. I want to put out as much vague info as possible so teams have to waste time on a game plan where the player might be there even if he is not. Also, I don't want the opposing players knowing the exact problem for fear they might try to cheap shot it and injure that player. This is probably more relevant to football and hockey but I can see instances where you wouldn't want to let the opposing team know your weakness in basketball as well, especially for big guys that have to bang under the basket.