Logging in just to read chat, follow people around etc. in theory there is nothing against this.
Actively working to help other factions has been a "grey area" but in more recent times ZOS seems to be very much against it.Mittens is that you?Spies in this game will never ever equate to the infamous antics on Eve Online, no offence to any wannabe spais here
Because there is absolutely nothing you can do to stop them.
It's not like you can shoot them to make sure they don't do it again.
I mean it's not like their character is in stealth watching them, or in disguise, pretending to be with the opposition, and can be uncovered. Their character really is, in game terms, with the opposition, so you simply can't touch them.
All the annoying nefarious crap like this is the bane of Cyrodiil. Boosting / spying / flipping / scroll running / emp flipping etc etc etc