I agree that Williams composes for the traditional "classical" orchestra. But he is an old feller, probably one of the last from a tradition of golden age Hollywood music that peaked from the 1930s to the 1960s. Back in those days, and reaching even further back to the late 19th century, the traditional acoustic orchestra was a staple for all sorts of popular music entirely apart from symphony, opera and ballet. But things gradually changed in the 20th century, first with electrical amplification, then the electric guitar, then keyboard synthesizers, and finally digital equipment. As late as the early 1960s, popular singers still were routinely backed by a traditional acoustic orchestra. As late as the early 1980s, most Broadway musicals still featured a full acoustic orchestra in the pit. But no longer. So I think it more accurate to say that John Williams comes from an old popular music tradition, which he represents quite ably.