I think that they have a list of approved ISPs or some dependable way to make sure it doesn't happen on metered connections.

http://windows.microsoft.com/en-us/windows-10/windows-update...

"As with Windows 8.1, Windows 10 won't automatically download updates or apps if it detects that your PC is using a metered connection. Similarly, Delivery Optimization won’t automatically download or send parts of updates or apps to other PCs on the Internet if it detects that you're using a metered connection."