As suggested above, in many cases, it's a matter of corporate agreements and perception. Many companies will classify hotels and either recommend or require that business travelers stay in certain classes.
My company, for example, does not have a prohibition on any type of property, but we are guided first toward our "preferred" properties (probably with financial inducements, negotiated rates, etc.), then "business class" hotels (anything from a Fairfield Inn to Grand Hyatt). We are generally discouraged from booking budget properties (Quality Inn, Super8, Econolodge), B&Bs, hostels, and so-called luxury properties (MO, Ritz, Peninsula, St. Regis, etc.).
There are some "gray areas" with hotels that some would consider "luxury" like JW Marriott, Intercontinental, Fairmont, etc. but might still be within policy.
On the whole, I'd say it's more about the marketing and the fact that these luxury properties are aspirational and they don't necessarily want to dilute the perception by equating with business hotels, although they may be business-traveler friendly. Rather, they emphasize their spas, fine dining, concierge services, and other such amenities.