I don’t think that’s right. Assets aren’t the only aspect of a program which can be protected under IP law. (Not that I take the law to be the paragon of ethical guidelines)
Ethics aside, the law is apparently pretty clear. The American Bar Association has an article discussing board game copyright[0], and here's what they say:
> Therefore, the systems or processes that make up the core of a game—generally referred to as the “game mechanics”—are not subject to copyright, even though the written rules, game board, card artwork, and other elements—often referred to as the “theme” of the game—may be. Game mechanics can be as simple as “roll dice and move a token along a track,” or far more complex. Regardless of the complexity or originality of a given game’s systems and processes, its game mechanics will likely not be protected by copyright.
What about patents? Surely if Amazon can patent a one-click shopping experience than game companies could manage to patent much more complex systems? Then again if they could then some of them probably would have by now.
Why wouldn't it be acceptable? As long as you don't steal assets you should be free to write any computer program you want.