On the website it does say that it can't be installed after delivery, but that's inaccurate. Tesla doesn't install it after delivery, as it's too complicated, but you could, at least in theory... if you can get the parts for the power and turn signal plug... and ask Tesla nicely to turn on the software side of it...
In the USA tow hitches are not very popular, because most people who have to carry large objects regularly just use a pickup truck.... and the weight being limited to 2000pounds (910kg) is somewhat disappointing, as electric cars can pull safely a lot more ... but that limit is most likely imposed by regulations, not physics.