I think they barred timed nodes on visited servers for technical reasons, quite frankly. The timer in question is likely being processed/tracked by your home server, so allowing timed nodes on other servers could potentially open up the ability to gather a timed node multiple times by visiting another server with another timer.
It's the same for hunts in a sense - every server has its own hunts. The difference between hunts and timed nodes, however, is that the players already on that server can kill the hunt before you even get there, essentially making hunts a shared resource.
Is it a smart design? Probably not. But you can at least see why they did it this way upon delving deeper.
