Referring to a previous post about nmap port scanning an isp ( forums.remote-exploit.org/archive/index.php/t-7364.html ), where thePrez98 said:

... 'You clearly do not own "more than about 4-5 domains" therefore you have admitted to scanning domains to which you do not have permission or authorization.' ...
I'm curious why the act of port scanning is illegal. Doesn't nmap use the same mechanisms an any web browser establishing a TCP connection, only in a different manner (ie couldn't you point a browser sequentially at every ip and monitor the response ) ? Do you not have permission and authorization to point a web these same ip s ? In other words, what makes nmap different?

Also, is the illegality a law or is it a violation of an agreement one signs with the isp?

I'm partially playing devil's advocate, though I'm genuinely curious about the definition of public vs private space on the internet.