It's a clear part of the social contract of innovation patents.
Society says:
- Ideas aren't things you can really own
- Copying stuff means more choice for consumers and lower prices.
- But copying stuff before the innovator has a chance to earn from their work is bad for innovation.
- If we agree an idea is novel and non-trivial (patentable), we grant the innovator the exclusive right to manufacture, to licence, or not to licence, the implementation they describe for a limited period of time, if and only if they describe it in enough detail to reproduce it.
- In so doing, we provoke further innovation as people try to achieve the same without infringing the patent.
That last one is the secondary outcome, not the primary intent of patent law.
The primary intent is to grant limited protection for patent holders to have a chance to earn from their work. "You explain how you did this so other people can do it in the future, and we will grant you the right to licence that idea exclusively for about half your professional lifetime."
Beyond that point, implementing the design is what absolutely should happen -- that is why the patent is basically education about the design itself. Implementing clones of expired patents is a social good. It actually proves the value of the patent system over the longer term.
If we call this behaviour -- if it infringes no other designs -- unethical, we're really arguing for corporate welfare based on goodwill. That isn't how it's meant to work.
Now, whether Behringer is unethical is another matter. There would seem to be enough other evidence of that!
But implementing expired patents is not bad, even if you literally start selling them the day the patent expires. Go for it.
There are similar issues with TTArtisan and 7 Artisan lenses. Yeah, expired patents of good lenses deserve to be implemented, updated, tweaked, and sold more cheaply. Everyone benefits. But do they have to hew so close to other people's design language too?