"Just a thought, but why should it be illegal to hack a website? The hackers are doing them a big favour in demonstrating that their security is lacking."
Would you feel differently is it were the website of the Australian Medicines Handbook which was defaced just as you appeared in a hospital Emergency ward? How confident would you be that a rarely-used drug listed as having "no major side effects" was actually so? How good would you feel during the minutes when the staff dug out the book and would you be hoping that the administrative staff had previously applied the online errata to the book, sometime within their well-known negative amount of time to in which see to the important after seeing to the urgent?
Or how about a subtle defacing of the Building Code, say halving the required load for roof beams. Let's say that's discovered five years later when someone questions the entry "133t" in the Index. Would you feel comfortable in your new home?
Sure, a lot of the web is used for entertainment, but some of it is used to inform real work. Sometimes that real work can have real world consequences if fed wrong or late information.
Then there's simple annoyance: having to ring the university and wait hours rather than check a website in seconds to see if you got in. There are laws against other annoyances, so what makes hacking special? Taggers are equally demonstrating a hole in physical security, but building owners don't view tags as a favour.