Historically the problem was that companies did not deploy outdoor signage in areas
that were known to be a potential theft or vandal risk; now these new protective TV cases are used in the roughest neighbourhoods imaginable, down to units being installed in Central Park New York.
Now with more and more people putting plasma and LCD televisions outdoors in their backyards, they also need to protect the TV from the weather and potential theft or vandalism.
It is like taking a standard computer and taking it to the dirtiest factory imaginable and leaving the computer there, only finding when you return in 2 or 3 days that it no longer works due to the fluid or dust ingress. Anyone can make a welded steel box, it is however the special electronics that go inside the unit that separates the specialists.
So the unit has to have some built in protection, so for example the protective TV case needs to be either IP65 (European standard) or NEMA 4X (US standard) for preventing water ingress. If you are looking for something to protect from dust, you have to consider a unit with an IP56 standard. The best option is to put a NEMA 4X unit out there and you know it is fine for all year protection.
Then you have to consider cooling as these screens give off a lot of heat, as well as keeping it sealed from the rain, snow and sleet. Then in the colder climates you have to have a heater otherwise the circuitry will corrode with the moisture inside the enclosure.