Microsoft has reaffirmed that Windows Insiders who clean installed the Windows 10 Preview can upgrade and activate the Windows 10 RTM for free, regardless of whether they have a Windows 7 or Windows 8.1 license. This announcement was made on Friday, and on Saturday was silently changed to remove any mention of being activated unless you upgraded from Windows 7 or Windows 8.1. This caused confusion, but Gabe Aul has again reaffirmed that Insiders who clean installed the Windows 10 Preview will receive Windows 10 for free, and activated.
Now you may be wondering why Microsoft went ahead and confused the situation by changing the way the announcement was worded in the first place. Well, in short, they did it to cover their backside. Soon after the original announcement, a number of media outlets began posting stories such as "How to get Windows 10 for free". This is not what Microsoft wants, and continues to advertise Windows 10 as being free for Windows 7 and Windows 8.1 users only.
It does appear that upgrading from the preview builds to RTM doesn't get you a valid license however. You will remain activated (and can clean install if you wish once upgrading), but you will not have a valid license. This isn't something normal users need to worry about though.