Whoa, what? Where did you read that from?
They talk about it in the books. Maggie made sure Harry was conceived/born under conditions to be able to fight Outsiders. Which is one of the reasons he's so freaking powerful at making stuff blow up.
We really don't know that much about magecraft in the Americas in the Nasuverse, but it has to exist. Native Americans for one thing had their own mystical practices and abilities, even though chances they call it "Medicine" and not "magecraft" and then there were probably European settlers who were magi. Not to mention, because the Association gives pretty much zero fucks about 'Murica, it wouldn't be too hard to reason that some mages with sealing designations ran across the pond.
There could be some stuff like the Native American Medicine Men and Women thought the handful of magi were abominations, witches, and against the natural order. Which, considering magecraft that's not too far off. The magi are like, "These heathens are fucked in the head!" because the Medicine People give zero fucks about reaching the root and use their abilities to help their fellow man and to protect nature's balance. With a few exceptions that are pretty much hunted down and killed when possible because Creator doesn't buy that shit. Hey
The Ravenmocker myth had to come from somewhere.
So then of course you get some of the sealed magi who still want to reach the Root go into hiding and try to do things to preserve their blood line. So there's a lot of inbreeding and even stealing Medicine People to freshen up the bloodline, improve Circuits, and make sure the offspring have a better affinity with American soil. This way, you get a lot of abominations.
Also, with the lack of an Association or Atlas presence in the States makes it a sort of haven for the Non-Ancestor Dead Apostles to set up shop and start chomping on virgins. Or for less morally inclined magi to start doing experiments like Kiritsugu's dad did.
So, to tie this back into the Nasuverse proper, there are a couple ways to do this. Post Arcuied's true end in Tsukhime. Shiki hears about that there may be someone in the North American continent somewhere who has the means to quell Arc's blood lust. Either two things could happen- a Dead Apostle could have set up a trap to kill Shiki in a far away land or, as someone pointed out, he runs into another family of monster hunters because hey, the Americas probably have their own share of Phantasmal Beasts too.
Except they're inbred, gun toting, not quite Neo Nazi rednecks.
Second idea for this is the rather simple, Loreli goes on a vampire hunt after getting word that one of the Ancestors is making shop in the States or Canada.
Third idea could be post Fate or even UBW. A community is being plagued by Magical Threat X. Be it just some douche magi, a Dead Apostle, or the aforementioned Ravenmocker. There is a fairly young Medicine Man/Woman who lives in this town, but is way over their head. However, they had traveled a lot over the past years as their journey as a shaman to see how mystic forces around the world work. When they were in London they heard about or maybe even met this ginger Japanese guy who can pull swords out of thin air with a hero complex.
Desperate, and because they saw that pretty much all other "proper" magi are jerks, find a way to contact Emiya Shirou and ask for help.
If this is post UBW-True, Rin could go with him and just boggle at all the differences between American "Medicine" and traditional magecraft. Not to mention, she'd probably find it fascinating that a majority of Medicine practitioners use crystals and gemstones as well. Also, Shirou's fascinated by the fact that the person who called him for help specifically helps others with their abilities and really doesn't use them for their own gain. Which would totally confuse Rin.
