How do they know if I use the premium theme on more than one domain?

Hi Folks,

I’m curious. Most premium theme set up their pricing based on the number of domains/websites that is allowed to use their theme. Take Thesis for example, their personal option only allows you to use it only on one website. However, who is stopping me in using it for my other websites or letting my friends use it?

I’m not implying that I am going to do it but I’m wondering what sort of controls are in placed to stop the above from happening.