Well....
The way you make moeny with open source is that you offer services, like the server they offer, install help, maintenance, etc..
Why it has to be free? Because they are using code that was provided free in their software.
That is how the virality in Open Source Software works. You use a significant component that is OSS, your software becomes OSS.
::emp::
Red Hat Enterprise Linux might be a good example. They make a lot of money updating and supporting open source software.