Actually we verify all the domain names where it's running to make sure it's runned by the same guy.
If we detect, that it has been sold or shared, then we suspend all the 20 sites ( and ban the license of our system.)
We can monitor all the licenses, and if our fraud-checker is notifying as that there are some risky, and suspicious websites, we manually check them.

And you asked " why ":
Because most times one guy runs more sites and advertise the sites on his other sites. This helps him getting new members, and he has to buy paid advertisement for only one site, and he'll have advertisement for others too for free.
Hope you understand.