It is blatantly, obviously clear to any truly thinking person that the problem is the choice, not the tool.
Though I agree that you are technically correct, I don't think that addressing the actual cause of the problem is realistic.
Take the following analogy:
You could say that bike thefts are a direct result of the choice of the thief not because some bikes are poorly locked or aren't locked at all, and you would be absolutely correct. But this does not imply that we should ignore the very real solution of making it harder for a bike thief to steal a bike by using a good bike lock.
Similarly, there is a very real solution of locking down the most convenient mass murder tool to make it much harder for the murderer to commit their crime. Does this solve the root cause? No. Is it effective? Well, look at the track record of any other country with a similar culture but restricted access to guns. (Canada, for example is inundated by American television so our cultures are about as different as the differences found between two states).
Sure the root cause would still exist. But when a person attempts a mass murder with a knife because that's all they can get their hands on, the death toll will be much lower, if any at all. If a person attempts a mass murder with a home made bomb, they might just blow themselves up trying to make the thing before getting a chance to use it on anyone. And let's be honest, there's a certain level of knowledge that a person would need to get even that far. Basically, there is no tool that is as good for the job as a gun is for murdering.
And you know what really sucks? Because there is such an intense and myopic misfocus on the choice of tools being used by many, nothing substantial is being done about the real problem, interpersonal, personal/group, group/personal and group/group abuse.
And you know what really sucks? Because there is such an intense and myopic misfocus on the real problem, interpersonal, personal/group, group/personal and group/group abuse, nothing substantial is being done about the choice of tools being used by many.