If those are the popular metal-cased bolt-down wirewound resistors, then the "50 W" rating is like unto the top-line power rating for power transistors with mounting holes: it assumes a specific case temperature (a function of the heat sink) to determine the power dissipation that will keep the internal temperature within safe limits.
Old-fashioned wire-wound resistors (with vitreous enamel or silicone coating and no mounting holes) were specified for a given ambient (air) temperature.
Even then, a wire-wound resistor spaced away from the board run at full rated power will get very, very hot, since the resistor materials can tolerate much higher temperature than can silicon.
(Guess if I learned that the hard way, by reading the datasheet, or the easy way, by touching one.)