What's immoral? The fact that a mass market product designed for entertainment based on a franchise you like and featuring gameplay you like (as opposed to games and franchises you don't like) is getting extra stuff added and, at least in theory, improved upon? Boo fucking hoo. I guess if someone decided to remake glorious Daikatana with better graphics, functional AI and fixed bugs, you'd be bitching about how it "ruins the spirit of the original"?
Regarding ownership, no one individual author owns Baldur's Gate, D&D, etc. This isn't like taking a piece of fine art, painting some extra stuff on top of it and re-selling it as "improved." We're talking about a product with dozens of authors, none of which have any individual right of ownership over its content, and whose work was done under contract for another business.
Regarding modders: while not paying them for their work is shady even if Overhaul did get an agreement from the modders themselves, it's not illegal. And sadly, standards for employment and paying workers are different in North America vs. the rest of the world. In North America it's all but expected in some sectors that you work for free, sometimes for years, before getting paid. Videogames industry is the same - nobody's paying people for the tens of thousands of hours in practice and training required just to get an entry level job.