Like cocaseco said, it depends on the business. Sedo was never meant to be a strictly US business, no the .us name was going to be a disadvantage down the line.
The other thing to consider is that sedo.com may have cost them as much as 40-50 thousand in any case, even before they has the company up and running. It's an attractive and short domain.
If the .us can be had for $500 while the .com is, say, $25,000 - I would consider whether my start-up could afford to spend 25,000 of our limited funds on a domain right away. The money might be better used elsewhere. If we end up being successful and need to shell out $80,000 for the .com later on, at least we'll be a bigger company with more funds we can play with.
But it's all hypothetical without knowing what kind of business we are discussing, what the target market is, how much funding they have, and how important the domain might be to their early success.