That is a matter of practicality of just enforcing the laws around protecting migratory birds. You can't possess the feathers of protected birds, and it doesn't matter how you got them.

Research the massively destructive bird feather trade in the 19th century if you want context for why the law is that broad.

Of course. If you find a bunch shed by a protected species, there's a good chance they're ill-eagle feathers.