One of the arguements against international derives from differences in governance models leading to uncertainty of actual performance for some of the less developed regions, primarily on the dimensions of accounting and shareholder rights.
So, since US companies cover the globe and are regulated in ways that match US investor expectations, it is likely one is buying something closer to what they understand. An international index contains some equities with murkier substance.
That said, global audit firms and International Financial Reporting Standards (IFRS, the global successor to GAAP, Generally Accepted Accounting Principles, the old US standard) are making hwadway to normalize the playing field. In Bogle's prime, global firms outside of Europe and Japan had very hard to determine worth, since markets were less effective at governance. This is changing, but still somewhat the case.