The part you're missing is that 90% of those kids will never make it in top flight football. So imagine the feeling when great big club axes you and you're 18 with no clear future. Happens all the time.Most of the people want to escape poverty. Getting your kid chance to go to the better system where he'll get more opportunities (rather than just football) is a chance not many will pass. He'll get better coaches, better schools as well.
Most parents want whats best for their child. There's plenty of examples of people moving from east to western countries in search for better life.
While it sucks for Polish football, its a great opportunity for the kid. Dude, at age of 15 being at the one of the biggest clubs in the world. Imagine the feeling.
Heck, my hometown club sold a 19-year old defender a while back. Makes a debut, plays half a season, goes to Italy for $5m. Yearly budget of the club is $10m, so you know what, fair enough, I can live with that. When they're poaching 15-year olds on junior contracts, what do clubs get? Bag of donuts and a handshake?
Plus if it doesn't work out, he at least made a professional debut here and proven himself, so he can always come back and still make a enough to become indepently wealthy.
It's not good for the kids and it isn't good for football anywhere apart from the leagues in 3 countries.