quote from page 231: "some matrix actions are illegal, making them more risky than legal actions. The Matrix was built with security in mind, but of course they couldn't make it hacker proof. The list of illegal actions is pretty simple: all Attack and Sleaze actions are illegal."
this is the canon, if you disagree you are free to argue that it should be different, just don't go about saying hacking is legal, because when it comes to fluff it is not legal, period.
Spider get away because they can, they have the corps backing them, covering them, take your pick, and corps spy on each other, they take as much illegal activity against each other as much as they can in order to get that extra edge, but that does not make any of those actions legal. its unfair? yes it is chummer.
"Of course, G-men, security spiders, IC, and other users who are officially sanctioned by GOD never rack up an Overwatch Score, even if they're misbehaving. Such is life on the Matrix. And in the meat world , too, Come to think about it" page 232