It's not the outcome of each action that is similar, it's the mindset of the one who is performing it.
The art of 'hacking', or modifying electronic things to do what you want, to defy the rules set by the manufacturers and those who control them, is indeed very similar to graffiti and all forms of street art.
It's the whole idea of not being content with things the way they were 'intended'. They mayor of your city telling you that you can't create art is just the same as Sony telling you that you can't modify your Playstation. It's YOURS, and you won't stand for somebody telling you that you can't do what you want with it.
Back in the early days of computers and the internet, people would create small graphical images and animations, some in the same styling as graffiti, to go along with their pirated software.
There's a lot more that is similar between the two subjects than you may think. I guess it's really a matter of knowing what the phrase 'hacker' and 'artist' really mean, and stepping outside the stereotypes projected by the media and the uninformed public.
I am an 'artist' and I am a 'hacker'.