So, it may not make much sense, but here is my guess as to why it was changed:
A large majority of players are from the US, specifically on this server and on these forums (people from other countries tend toward their own area's servers). In the US, there is a lot of dislike of Bin Laden (as you might notice). Now, while it is true that rationally, this SHOULD mean your name was A-OK, it still referenced a very bad man, someone who is approaching the same level of hatred in America as Adolf Hitler. They wouldn't let you use a name about Hitler, so they don't let you use a name about Bin Laden.
I get that saying he's dead isn't 'supporting' him, but the reason they wont let you have your old name is because they don't want any summoner names that reference these bad men, else they might, by some person somewhere, be construed as supporting that same bad man.
I know logically it doesn't make much sense that saying someone is dead could be considered supporting that person, but that is my guess as to why.
That being said, IMO, you should get your old name back.