As Scott says, there have been towbars fitted to ST's before, and I have seen that picture of one pulling a trailer. However, the ST isn't designed to. From what I've read, the problem is that the bumper is too low and because it's been lowered with stiff suspension, then if you pull a caravan or a trailer then this makes the back end go too low (it could pull up the front) and it's considered dangerous.
In the Fiesta manual it lists the tow weight of an ST as not applicable, and this is because Ford wouldn't supply or fit one, although of course you would be able to get a custom made one. If anything went wrong however, then the warranty, if you still had one, would be invalid.