Let's try to look at this the way a testing lab might....
We first need to make certain assumptions as to who may be using the thing, and the conditions under which it might likely be used.
Something intended for household use, all the time, by the uneducated, and where children are present will be treated differently than something that will obviously be used only in certain conditions by trained personnel.
Then we look at how standards are developed. There is no all-wise "Czar" who decrees what the standard will be. There is usually some sort of "consensus" approach, with input from other standards, groups, the industry, the manufacturers, and sometimes even consumers.
As far as UL is concerned, the NEC is but one of the codes that they consider- and the application of the NEC to portable or emergency equipment is minimal at best.
I can see assumptions being made that a portable generator, or welder, is likely to be used by folks with some sense, for brief periods. We also have no way of knowing the environment in which it will be used. Sure, it COULD be used outdoors in the rain- just as it COULD be used to power a fire pump- so should we also require it to meet fire pump standards?
Bubble covers are not required in industrial locations that are subject to frequent wash-downs....as there is both the risk of damage to the equipment, and the assumption of some sense on the part of the user. I think the same logic applies here as well.
