Because it's totally balanced to give the tankiest PvP class a strong damage deflection that can instantly do up to 5k retaliatory damage at once...
This doesn't have to affect PvE at all. If they adjusted it to do one retaliatory attack per damage/combo taken and one for all debuffs applied rather than one for EACH debuff applied, it'd go a long way. In PvE debuffs like that are rarely a factor in StA damage/hate production anyway.
Or it could be adjusted as an AoE taunt like a Conq's Retaliation and have the damage limited to once per second per target.
I am aware that you can do a white hit every 3 seconds 3 times to remove the shield without taking much damage, but I think it's silly that a class should have an ability that makes them untouchable for 9 seconds without killing yourself. ESPECIALLY on a Guardian.
Just because something needs to be adjusted to not be totally imbalanced and broken in PvP doesn't mean it's going to completely break the class guys, relax. And BTW, we have Guardians who tank T4 no problem, maybe you guys just need to work on your gear/rotation/spec a bit more :P