As i was saying above, the majority use-case of update sharing is sharing updates over a LAN to other computers in the same office.
Note that, even in that case, the connection to the outside world might still be metered/capped—but the connection to LAN peers obviously isn't. That means that "does this traffic cost anything" is an evaluation the OS would have to make per socket (or requested socket from a higher-level library, like Windows' BITS), rather than per interface.
Note that, even in that case, the connection to the outside world might still be metered/capped—but the connection to LAN peers obviously isn't. That means that "does this traffic cost anything" is an evaluation the OS would have to make per socket (or requested socket from a higher-level library, like Windows' BITS), rather than per interface.