Why is hacking cable modem illegal?

Hi, im in Australia and have been reading up on people who have hacked various cable modems for better speeds, which also comes with topics about people being sued due to such behavour.
Me, being young and naive, wants a bit faster net- thinking of hackin it. but just a bit concerned that optus will do something- dont know what theyll do, though.

is there any legal issues here?
anyone hacked a cable modem before?
have you get in trouble for it?
Does anybody know why hackin a cable modem is illegal?
and ofcourse why would ISP's care about something as little as this.

 

well its nice to play around but this is a really but hole thing to do, your neighbors basically get bitched
http://i'manidiot.com edited by ddp the only reason i show you this guide is because most on the internet are made incorrectly, this one on the other wars written well

and yes you are in violation of contract you have set up with your ISP, it's like getting 100$ worth of service you didn't pay for