They gain big benefits by having a single pool of datacenters able to serve users from anywhere in the world. If they needed to guarantee that an EU user would always be served with a machine in the EU, I can imagine it would add at least 20% to their operating costs.
They'd need more equipment both inside and outside the EU to handle failover, maintanance, etc. They'd also have more complexity slowing development down (they can no longer have small services 'mastered' in just one region). And there is substantial extra complexity in application design (what when a tweet from an EU user is retweeted by a US user, but then replied to by an EU user. Where will the text of the tweet be stored? How will deletion be handled?).
For example, will HN have to have seperate databases for "comments by EU users" and "comments by US users"? And will they need a process to migrate your account from one to the other?