You don't have to pay a cent and I assume this is the full windows 10
Your assumption is wrong. In the last round of the free Windows 10 issue, Microsoft cleared this up pretty good:
- Genuine OEM and retail licensed Windows 7 and 8.1 installations can be upgraded to Windows 10 for free. Once the Windows 10 upgrade is performed, clean installs can be done if desired.
- Volume licensed installs will not be entitled to the free upgrade, customers will receive upgrades according to their service agreement.
- Anyone* can install and run preview builds of Windows 10, and the Insider program will continue to issue preview builds with new features and developments. Preview builds will expire, users of the Insider program will be required to regularly update to newer builds.
*
There might be an exception for business use installs, I have no idea
TL;DR: You can be a beta-tester (run preview builds) for free, or pay to use the stable builds. Whether you go to a preview build from a pirated Windows installation or nothing doesn't matter, you get it for free with the quirks of the preview build.