Hackers argue that gaining access is a learning medium. An if there is no intent to act maliciously, there is no harm in attempting to further knowledge. There are too many other accepted situations where this is widely accepted. I was given a computer at a very early age and allowed to do whatever I wished with it. By being given this freedom, I learned more about computers than I probably ever would have learned in a class.
People teach themselves to play instruments and read music without guided instruction.
There are many cases where this is true. If we were to say that hacking is unethical we must also say that any of these forms of self education are likewise unethical.