Its just like this: you can upgrade to Windows 10 and if you stay on the Windows Insider Program you will have an activated Windows 10 with the cutting edge features (and headaches). So you are licensed to run a Preview Build, but you must buy or otherwise aquire a retail license if you do not want to stay on the Insider branch and want a RTM version instead.
So after Windows 10 goes RTM the Insiders will briefly be on the RTM version as well, and they will move on to newer versions.
And the MS Account is only required once to upgrade after build 10159 I understand, to mark your Windows as licensed via the Insider Program, but you can go on use a normal, local Windows account as long MS doesn’t remove that ability for everybody (I assume that will not happen).