There are supposedly two points where you have to confirm you want Windows 10 installed under the new update scheme. One random dialog box that informs you Windows 10 will be installing at some point soon, and then an agreement with the TOS after Windows 10 has finished installing but before it first boots up to the desktop.
Some have reported clicking through these because they don't really read things. Others have said the process is bugged and they did not get either, Windows 10 simply installed like any other update and they returned to a computer with Windows 10 up and running on the new desktop.
As Sittingbull said, you can roll back if it has been less than 30 days as Windows 10 does have that safeguard built in, in case of unforeseen compatibility issues.
http://windows.microsoft.com/e...dows-7-or-windows-81 Gives the details on how to roll back.
As for lawfulness, Windows 10 is considered an update to the Operating System by Micrsoft, and you only get the auto-upgrade if you have your previous version of Windows set to automatically download and install Recommended Updates. So... who knows?