I wouldn't say that it's a matter of "justification," but a matter of what the customer desires. Both guns are perfectly functional, but they serve different markets. The Express is a low-cost option intended to give people a functional gun to put in their closet, under their bed, or the behind the seat of the truck. The Wingmaster, besides the things you mentioned like forged parts and metal trigger guard, has a gloss-finished fancy walnut stock, fleur de lis checkering and even comes in engraved and gilt-inlayed special editions. It is probably the most "elegant" pump action shotgun in production today. It generally sells with long barrels for things like upland bird hunting. Most people buying it are turning down higher-priced over-unders rather than stepping up from the Express. But the barrels are interchangeable with shorter ones and it can be converted into a "combat" style gun.
The 870 Police model is built similar to the Wingmaster, but with a more utilitarian stock and foregrip. Both the Wingmaster and Police share the forged parts, metal trigger guard, and closer tolerances on the slide action. Civilians buying shotgun for something like home defense or the camp, probably don't want to spend more than for an Express. If they were willing to maybe because they thought they might shoot it a lot, maybe take it to combat shotgun classes or something, and they wanted a high-end pump-gun instead of a semi-auto, would either get a Police model or a Wingmaster and then change out the long barrel and fancy stock for a shorter one and a maybe a synthetic speedfeed stock or something. If they were going to "accessorize" it with a shellholder and weapons light, rails and all the mall ninja stuff, they will probably covet to start with Police/Wingmaster internals.