Do Windows Insiders get a free license?
* This is a simple question, with a complicated answer. The short answer is No. Neither do Windows 7 or 8/8.1 users. They get a free *UPGRADE*, which is not the same thing as a free license. The *UPGRADE* upgrades an existing valid Retail or OEM Windows 7/8/8.1 license to entitle you to use Windows 10. Just like any other single license, you can only install one copy on one machine at in given time. That one copy can be Windows 7/8/8.1 or Windows 10, but not both at the same time, and not both on different computers. (See section on dual booting and virtual machines).
* The longer answer is "sort of, but not really". If you stay in the Windows Insider program, you will continue to receive what are, for all intents and purposes, "Trial" versions of Windows 10, which expire after a period of time. These builds are Beta and Test versions, so may continue to have problems because they are not "Production level" code. New builds will be released periodically, and you must continue to upgrade to these new "Trial" builds in order to retain a working copy of Windows 10. New builds will be released before the expiration of the old builds. There is no information on how often these builds will occur, but some hints have been that they may come as often as a monthly basis. Perhaps even more frequent on occaion.
The primary difference here is that if you leave the Windows Insider program, you will no longer have a valid license to use Windows 10, so you must acquire one if you want to stay on Windows 10. You can acquire a free valid license either by upgrading an existing qualifying version as specified above within 1 year of the release date of Windows 10 (July 29th), or by purchasing a license for Windows 10 (or through MSDN/Technet or other standard ways). Either way, to stay on Windows 10 you must have a valid license, since one is not given to you simply by participating in the Windows Insider program.