Why is it important to shop for SEO USA based firms? Well, it isn't always. It really depends on what you are shopping for. If you are well versed in SEO, then you probably already have a plan, know what you want to do and just need extra hands to execute. In these cases, using off-shore teams for certain aspects of an SEO campaign can save you money. However, in terms of overall SEO campaign management, that's when its time to look for SEO USA.
SEO firms that are based in the USA will have some distinct advantages over off-shore SEO firms. Specifically, there is no language barrier. Secondly, you are likely to be in the same time-zone. Finally, there will probably be a shared sense of which search engine marketing activities are appropriate. If there is any consultation needed, you will find a USA-based firm to be far superior.
That being said, there are also certain manual tasks that an SEO firm in the USA would just cost more to have done. Repetitive, data-entry type tasks are perfect for sending off-shore. Many coding activities can be successfully sent off-shore. For these types of activities, it doesn't matter who does the SEO. USA firms will have to charge more because their labor costs are higher.
In my opinion, the best approach is a hybrid. Find a SEO USA-based firm to quarterback your entire search marketing effort. But make sure they use off-shore resources appropriately to keep costs (and therefore prices to you) low. But no matter what, make sure they are accountable, work with Fortune 100 clients and offer full reporting tools so you can see what's happening at all times.