Details meters.
Weak auras.
DBM/Bigwigs.
Warcraftlogs.
Battleground timers.
All of these addons parse the combat log looking for a trigger.
Why not spy? You dont need to dl spy to watch combat logs for players in the world, or anything else that is listed in the combat log you can create a simple weakaura to do that for you.
Spy (i dont use it, but i might install it just because) watches all combat log entries in the area, not just for rogue stealth protection - maybe people want to use it to go find players to hunt aswell.
You cannot break /target macros.
You cannot stop the combat log showing the name of some thing that did something.
You cannot stop a ui window that keeps track of said players, with range limits on them (either in range or not) and creates a button that fires the /target macro.
If you did it would break every aspect of the rest of the game and how its been played for the last 15 years.
Think of spy as a viable, always running, arena target frames addon.
Think of spy as the DBM of Classic PvP.
Think of spy as just another tool at a players disposal, and if you are a skilled rogue you would already expect the players to know you are there and learn to adapt