Lucid is right - basically, because the source code is open, some Prof-UIS files are available on the CVS. However, you cannot download these and go and use them in other C++ projects. Since none of these files are at all used in neither the runtime nor the plugin SDK (which deliberately hides all UI code) then you don't need to worry about Prof-UIS licences at all if you're distributing a game or plugin.
In retrospect it was a bad decision to go with Prof-UIS - since we have to pay for upgrades, which is always difficult for an open source project. But since we made the decision MFC (a free library with Visual Studio) now has the same features - so in Construct 2, we'll definitely be using that.