So fresh off a bunch of public domain jokes

I think there are about 5 people who send me stuff like the 4 or 5 posts of random smurf I dropped on here last night before the (hopefully comedic) post about Super Duper...well, you know the rest. I have often wondered something.
Virtually every company in the world (including mine where at least two of those people work as, obviously, do I) has an e-mail policy that strictly forbids that sort of behavior. Seriously, how many of us have signed something to the effect we will not use company e-mail for non-work related stuff? And how many of us follow that?
After all, at least three of the things posted below I have seen a minimum of two times. Following the trail backwards I see where often upwards of 30 or 40 people are included in the path of forwarding. Some of the jokes I have seen so many times I can tell you about half of them before I even open the e-mail just from reading the title.
In my humble yet accurate opinion these things are actually a somewhat valuable use of the system. After all, happy employees work harder and are more productive, it says here *points to script*. And anyone that knows me knows I love to laugh. In fact, I think I shall do so now. Bwooohahahahaha. Oh, that's better.
But back to the point; at what point is it in the companies best interest to crack down on these things? Naturally, I would argue that most people are wise enough to only partake of them when they have spare time, but inevitably someone who is there simply for a paycheck and has no conscience at all and no desire to do their job abuses the system. This brings the wrath of CEOs down on people for whom it is probably a beneficient tool for relaxation.
Of course, in our legal system, if they only censure the abusers, those same people then sue the company for selective enforcement, so even people who make reasonable use are in trouble. That, of course, gets us into the whole murky realm of a subjective judgment. What is "reasonable" and what crosses the line.
The answer is clear; oh, wait, dinner is done. Later.