Transfer?
It’s not a design flaw – that would imply that Anet were trying to make it work and screwed up. It’s a limitation of the technology.
This may not be a completely accurate description but the way I understand it is that all the EU servers are together in one building (probably one room) and interconnected – so guesting from one to the other is basically playing over a local area network (LAN). And it’s the same for the NA servers.
But those two groups of servers are in totally different locations thousands of miles apart (Germany and Texas respectively, if I remember right) so guesting from one to the other would require your computer to connect to the server over the internet and then the server to connect to the other server over the internet, with some data stored on each. It would drastically increase lag and demand on the servers, meaning if multiple people did it everything would run slower for everyone.
We’ve already got a better system than most MMOs I know of, where servers are totally separate and there is no cross-over at all.
“Life’s a journey, not a destination.”