I think it’s pretty clear that the BRF have tapped Harry and Meghan to have their focus of work veer somewhat international/Commonwealth going forward. They have both been given Commonwealth position/offices, which seemed to be hinted at when she made such a point of having all of the Commonwealth represented on her wedding veil. That focus builds quite logically on his pre-Meghan history of African volunteer work and the Invictus games. The BRF is not simply the royal house of the UK. The queen has always placed great importance (moreso, I think, than many of her British subjects, especially those I see commenting here) on the role that her household plays in Commonwealth countries and the opportunities they have to make a difference on a global scale.
If the Sussex foundation is going to do work that tackles some of the most pressing issues in the poorest Commonwealth nations, they are going to need a lot of donations from all over the world. And if they want American corporate donations, or a seamless partnership with like-minded charitable groups with US headquarters, they need to be registered in the US as a 501(c)3.
Including the fact that the Princes Trust, Cambridge foundation and many other orgs and charities do this to protect themselves from liability. The organization I work for does this too. Its standard across sectors, entities and organizations. In addition, merchandise is actually an important and sometimes good way to generate revenue for an org and has the added benefit of helping spread name recognition for a charity. Many not for profits, NGOS, charities have merchandise available. So it wouldn't even be that untoward if that was the plan. 
