Given that AWS US can still easily 'control' any data it wants to get from its Euro subsidiary (just by pushing a new code version that has a small change to make that data accessible), I don't understand how AWS US would avoid being compelled to turn over the data in a high profile case. I.e. they could always implement a backdoor or make a targeted modification to the AWS code base that would allow them access if they wanted. Unless AWS EU does a full code review on every line of code being changed in the underlying AWS codebase, they'd never be able to stop it.