Well, it's because you will have a criminal provision that is completely uncertain in its meaning. The people who are out there on the street will have no conception about where the line is between things that are okay and things that are not. You don't have a check on political decisions, as Professor Alford just alluded to. You basically have an open book.
If you have the political wherewithal or the means to direct certain outcomes, certain prosecutions or certain initiatives, then of course that's what you'll do. That is a poor way to construct the law. You are asking for trouble instead of trying to design the law so as to prevent the possibility of that kind of outcome.