JK Rowling did not 'buy' her way in. Her home publishing company will have sold her books via foreign rights to an overseas publisher (or overseas arm of her home publishing company). This is a case of two companies doing business together, rather than a person representing and conducting business herself.
I'm not personally acquainted with her living arrangements, but it would appear that she resides in the UK. So it's perfectly legal for her to sell products to the US as she is in her native country when doing so. However, if she were in the US on a visitor visa, she couldn't just set up shop and sell only to the UK to avoid breaking US immigration rules.