Well I already know English, but really the classes are of highly worse quality if they are taught in English?
Considerably so. They are a cash cow for the university, nothing more.
Really not even from the school itself?
Why would they give scholarships to students who are studying in English and paying for it? It wouldn't make any sense.
Don't Polish schools want to be able to say that they have students from the U.S studying within their campus?
Why would they care less, as long as people like you keep attending?
In the U.S foreign students get scholarships so the school can say they have students from over 25 countries attending.
Means nothing here. Polish students couldn't care less if there are people from all over the world attending or not. In fact, in most universities, they hate those on English language programmes because the English language programmes are notoriously easy compared to the Polish ones.
Would going to the Best school in Poland really be worse than just going to an alright school in the U.S?
Yes. Jagellonian is 351-400th for the Polish programmes, not the English ones.
English language studies are mostly a complete joke in Poland, existing solely so that foreigners can buy a degree at a low cost.