fritz,
That's a very good general rule, and consistency is most important. If it's a foul against one team, the same foul should count against the other team.
But in the interest of being consistent, why wouldn't a foul in the first half not be a foul in the second half? Why wouldn't a referee select to call a foul in the penalty area in the manner that he would call the same foul at midfield?
There is a great deal of subjectivity involved while a referee is trying to be objective.

As you know, it's not easy. Golden rule: Be fair, be consistent, keep the players safe.