The issue is owning companies that are based abroad, not owning US companies that derive some of their earnings from abroad.
As mentioned by Louisville in a post above, it is all about the efficient frontier. By owning assets in US and abroad that are uncorrelated you maximize return and minimize volatility by owning 70% US / 30% international. See
http://www.bogleheads.org/wiki/Domestic/International for more discussion. The 70/30 split is what most people use. The 70/30 split varies with time.
Again, as mentioned by Louisville, there is a lot of discussion about whether or not international stocks are really uncorrelated anymore since there is so much global trade and interrelationships between countries.
Regarding 50% of the market being in the US -- that means 50% of the market is outside of the US. Why ignore half of the global stock market?
Here is some really good data from the bogleheads link above:
Consider that in international markets you will find...
7 of the 10 largest automobile companies
7 of the 10 largest diversified telecommunications companies
8 of the 10 largest metals and mining companies
6 of the 10 largest electronic equipment and instruments companies
5 of the 10 largest household durables companies
Pretty compelling reason to invest abroad, I think.
Edit - I was just playing around with the efficient frontier plotter on PortfolioVisualizer and the efficient frontier curve is much different depending on what time period you choose. The Bogleheads article uses 1970-2008 and that gives you around 70/30 split while maximizing return and reducing volatility. However, if you look at 1992-2014 then you can minimize volatility by using 80/20 split but CAGR decreases from 9.43% to 8.83%. So, for this time period you don't minimize volatility and maximize return simultaneously.
So, it looks like the minimum volatility (risk) point is about the same (70/30 to 80/20) but the maximum return point shifted from 70/30 to 100/0. Here is where the crystal ball comes into play: what will the efficient frontier curve look like for future time periods?
See
https://www.portfoliovisualizer.com/efficient-frontier?s=y&mode=1&assetClass1=TotalStockMarket&startYear=1992&verticalAxis=2&assetClass2=IntlStockMarket&fromOrigin=false&endYear=2014