That's why I listed it a rule of thumb; it's probably okay to place an Aspect on someone (the Love Potion example in the books) but players shouldn't assume that it's automatically okay to do so without consequences.
If there's a Law, then there should be a general understanding of what that Law allows and prohibits. Again, that becomes a group decision; some groups might be fine with some things that other groups condemn with extreme prejudice. If there's no way to guess IN ADVANCE that a particular act is illegal or not, you don't have a law.
In real life, people get jammed up all the time for actions they thought were legal, but weren't. In literature and games, that kind of arbitrary actions leads to hard feelings. By the same token, in real life most people have a pretty good idea (a rule of thumb) as to what's illegal; they get in trouble in areas that are unfamiliar to them. ("Okay, I knew it was illegal to break into the house, but you're saying I'm breaking the law by shoving stuff through his mail slot?") If the Laws of Magic are imperfect representations of metaphysical laws, rules of thumb become even more common. ("Okay, I know I can't jump off this roof and survive...but I think I can jump across to the next roof to safety.") And again, it's where characters are guessing, or forced to make split-second decisions, that the problems arise.
