I'm giving you the benefit of the doubt that you're not trolling. If you are, you should be very ashamed of yourself, as I'm wasting very earnest and well meaning words here.
Foremost, this is not a strictly USA phenomenon;
Anecdotaly, almost every FS posting that originates in the UK, is UK only. Granted this is a smaller number of posts than the majority which come from the USA, but as a percentage, the UK seems to be less interested in international business. Probably because their taxes can get so ridiculous.
As what I presume is our second largest user base, they have the luxury of being discriminatory in their selling practices without cutting themselves off at the legs. Those in even smaller markets - such as specific countries within eastern asia - would be unable to sell anything if they were so restrictive.
In conclusion, it seems to be that if you are in a large enough market that you can probably sell your item without going outside your country, most people prefer to do that.
Now, to consider why;
I'm fairly certain those two sum it up. It has nothing to do with racism or international bias, and has everything to do with expense and convenience. A transaction conducted within your own country will always be faster, and less complicated, than one that crosses international boundaries.
Shipping is slower, and if it's not slower it's much more expensive. For a seller advertising a price with shipping included, their $10 cost to ship within the USA just became at $25 cost to ship to their buyer in Japan, so they've lost $15 off what they wanted to get for the item - or they pass it on to the buyer, who feels jilted because the price went up just for them being foreign.
Worse still is the nefarious evil that is customs. All of our person-to-person trading is theoretically legal, within the country that you live, but it can quickly become "grey" in the eyes of the shipping or receiving nations customs officers.
Nobody wants to be involved in a transaction where the goods are seized by customs and held ransom for some exorbitant tax fee. The buyer is obviously upset because they paid for something they didn't get, and now have to pay more than they planned - and if they aren't blaming the seller for "disguising" the item better, they're wanting their money back for something they didn't get and may feel is the sellers fault.
Meanwhile, the seller is either forced to give back the money, and then try and get the item returned from the clutches of whatever customs agent decided to "confiscate" the goods, or they're out an expensive item AND their money.
Meanwhile, the for sale thread turns into what amounts to "negative feedback." This is a largely unprotected marketplace that goes only on the basis of goodwill, trust, and its only penalties are in public damage to someone's reputation. Nobody wins when this kind of thing goes wrong - both the seller and the buyer are likely to be turned down in the future based on obvious evidence of a botched transaction in the past.
In conclusion, or TL;DR - it's not racism, its risk from customs and inconvenience of slow shipping. Person to person international business is rife with both.