Although this is generally true, it is not a requirement. Some older panels have the breakers mounted vertically with the busses horizontal. I haven't seen newer ones that way for awhile,tho.
Just about all domestic panels here have a horizontal busbar (usually at the bottom) and breakers or plug-in fuse carriers mounted vertically in a single row (only one hot leg remember).
Three-phase is generally two vertical rows of breakers mounted sideways with a main switch at the bottom.
So does this just prohibit mounting it with the back against a floor, ceiling, and anything that isn't a vertical wall? But not which way the box itself has to be oriented?