Err. As far as I know they pass hack tests because their IVs actually are generated by the game's random number generation. It's just considered "cheating." I've had friends (not naming any names here... >_>) who have used hacked parents and traded the offspring and they were never caught. However, if you trade things from hacked parents and people find out and you didn't inform them, they'll be really mad and it's against the rules I believe.
All of this is just "as far as I know" stuff so don't take my word for it. =x
All of this is just "as far as I know" stuff so don't take my word for it. =x