Think he himself is the one who chose to use the name?
Bruno himself has explained that his name was a "damned if he does, damned if he doesn't" scenario.
(Leaving aside the fact that, as the son of a Senna, he is more than entitled to use his mother's surname...)
If he didn't use the name, he knew that people would make comment, or even assume that he was ashamed of his uncle. I can only imagine what some of the evil vultures on here would have said about him. If he did use the name, he was going to be placed under enormous and unrealistic pressure - which is exactly what's happened.
Add in the fact that Bruno's own father was killed only a year after his uncle and I can't even begin to imagine the level of emotions involved in choosing which name to use.
And if you are going to criticise Bruno for using the name of Senna, then you should also be criticising Ayrton, Fernando, Sergio, and any other of that Latin style where a child is given two last names.