Hi 800ftw:
You are right to assume that a high school student from the States OR another country that has gone to the Worlds or Olympics will get a full scholarship, If and that is a big IF, the school has money to give that athlete.
It is very rare for a male to have a full track scholarship; if he does, he is an absolute beast.
In the US, the very best high schoolers - I am taking Top 10, are almost on another planet with regards to their performance compared to the average kid on a track team.
This year (2016-2017) alone US high schoolers (not citizens - Mondo D is included in this group) can compete against the best Pros on the planet and . . . WIN !!!
Mondo - in pole vault had the WL for a minute
Sydney - in W400m, W400mh; she runs 54 secs.
How many UK School Boys were in the Track and Field (Athletics) Olympic Trials Finals; how many made the Rio Team?
These kids (the two aforementioned US high schoolers and the "UK finalists") get FULL scholarships. Most schools and if they have a good Director of Ops. will need to find another way.
As I mentioned before, at the "bigger" schools, salaries are tied to the performance of the team. They have KPIs and stats. to present and justify to their Athletic Director.
Having a national team member, an Olympian, or a World qualifier is an excellent recruiting and intimidation tool.
Example: If I told you that you had to run a 4X100m relay against Jamaicans. The first thing that is going to go through your head is "how fast do they run", "I hope we don't get embarrassed", etc.
Example: We just recruited a bunch of hammer throwers from Hungary. Only a handful of States throw the hammer at the high school level. If you are a rivals throws coach who is not truly confident in his/her ability to recruit and coach throwers (foreign or domestic). You have just "cost" your team points in the conference meet (the Americans score their meets).
Example: We managed to sign the Jr. National Champion from the Czech Republic in the javelin. (About half the states throw the javelin). Jan Zelezny is from the Czech Republic. The assumption being that they have "mini" Jan.
These examples all play into WHY foreign athletes are recruited by US Colleges and Universities coaches.
From the athletes perspective you can run in "sunny" California or live in New York City (trite I know), get a great education, access to great coaching/sports medicine, facilities, and get cool Nike/Adidas/Under Armour swag.
I can tell you this, if you were one of my athletes I would have tried to get you to run in the States. Think about this, let's just say you were on Texas A&M's track team last year as a walk-on or a partial scholarship. How much better would you be constantly having to try and keep up with Mr. Brazier and co. You would go to every practice KNOWING you had to "bring it". This is why the NCAA is the best, in my opinion, regarding U23 track and field competition.
Sorry for the long post.