The feign death penalty has pretty much been covered, but here's the rest of it.
You only get a FD penalty in a group if you do 50% or more of the total damage to a mob. It does not appear to cause the same penalty as if you do damage outside of a group without feigning. The penalty gets worse the more damage you do. It doesn't have anything to do with preventing low-levels from getting the killing blow and massive xp, because zoning or camping can accomplish the same thing. If you are flagged to get experience for the fight, you'll get experience in proportion to the % of total damage you dealt. This is what prevents low levels from getting massive xp from the killing blow.
Ultimately, you can happily blame any penalty incurred from FD on your group for the following reasons.
1) You FD because your tank can't keep aggro. Not your fault.
2) You FD because you're not getting healed. Not your fault.
However, if you don't know what /assist does or you just like to hit feign death for fun, it's your fault and you should stop being a jerk.
edit: This is true of all classes with FD, although a shadowknight has never done more than 50% of the damage to any mob ever.