Getting the law (or even policies on prosecutorial practices) changed to prevent another such incident is a VERY HARD task. Taking small steps toward that task is NOT "meaningless".
The victory to be celebrated here is NOT that the problem has been solved. It is not even that the White House now has to help work to solve the problem. The victory to be celebrated here is that the White House actually has to say something. (And they DO have to... if not, then the press coverage will increase until reporters demand an answer on their own. Heck, refusing to comment would be GREAT for our cause.)
Do you have any idea how difficult it is to make the White House at least make a statement about your issue? It is something powerful lawmakers and world-spanning corporations often find themselves unable to accomplish.
Now, that's not much: the White House's response may consist of meaningless blather. But it is not nothing: the White House has been forced to respond. And it helps to increase the momentum toward taking some kind of REAL action.
The petitions at WhiteHouse.org don't fix problems, but when something is NOT being discussed by lawmakers, this kind of thing can help get that conversation started. Belittling it isn't helping.