Also, "cloning" is a poor word for such activity. Writing code is hard and time consuming even if you're merely replicating functionality of an existing product. To me "cloning" implies that it somehow cheap or easy and it just isn't.
Not all the time to be honest. Creating a finished, complete and useful product can involve a huge amount of prototyping and several redesigns. You have to try lots of ideas that don't work, throw code away, redesign the UI as it becomes more clear how the product is used, test on users etc. If you heavily base your product on an existing complex one you're likely taking a big shortcut.
Crossing the line is using other people's work and calling it your own, regardless of open/closed source.
I think that's fair enough?
If in society it were made clear that copying everyone else's designs and market research were okay and to be sought when possible, then there would be little wrong with an open-source project copying the designs of a proprietary project. On the other hand, if a proprietary project is not allowed, in practice, to copy the designs of its competitors due to fears of legal problems or public backlash, then it would be unethical for an open-source project to copy other projects' designs since that would give the open-source project an unfair advantage.
At the same time, there is still an element of exploitation if in practice the only ones investing significantly in market research and R&D are the proprietary firms. After all, any time someone is reaping the benefits of someone else's work without giving back equally in one form or another, there is a type of exploitation going on.
Now, we should avoid thinking in black-and-white. We should avoid saying x is ethical while y is unethical. In the real system that is society, there are many variables involved, and these variables are floating point. You would need to plug all the values into the formula of ethics to really see the relative balance of ethics followed by any given project or company at a given time.
I can tell you from my own experience with (embedded software/hardware) R&D that coming up with a fully functional, reliable product in terms of design and approach is a very costly operation. Simply measuring and cloning someone else's product is worlds easier than designing it in the first place. After all, the people doing the R&D are filtering out all the bad ideas for you -- and most ideas are bad ideas, often for reasons not clear from the start.
Like I said, however, one of the most important factors to consider is whether everyone is playing by the same rules. Doing things your competitors are not allowed to do without getting into trouble is cheating. And cheating is unethical -- despite the inner animal in us thinking there's nothing wrong with cheating as long as we're the one benefiting. Remember: The question is not whether you can do it, but whether it's ethical. These are not the same thing.
If your clone would be used in a competing for profit restaurant, I would answer "yes, it is unethical" in all 3 cases (getting inspired by a dish feels different to me, and can easily be ethical, depending on scale and similarity. Getting inspired by 20 dishes from a single restaurant? Unethical in my book, unless there is a truly tremendous lot of inspiration involved.)
The world, however, sees things different for software. For example, you can legally clone the PC BIOS or fonts, as long as you only look at its outside. That is like cloning dishes from a restaurant based only on looking at what they present you when you buy the dish (= without visiting the kitchen, quizzing the cook, or dumpster diving to discover what the ingredients are)
But that is legally. For the first, most people would agree it is ethical. For the second, you'll find differing opinions.
Now, the open source angle adds a twist that makes my answer "it depends/I wouldn't know". Somehow, cloning, say, a product that isn't for sale anymore or doesn't run on modern hardware, and was produced by a huge company feels more ethical than cloning a $5 product created by a single developer who makes just enough to live from it from that product. In either case, cloning feels more ethical to me if it is done for one's own use, more so the less wealthy the cloner (cloning for own use, AFAIK, is legal around the world. For example, you can create a Rietveld chair for your own home, but not sell it or give it away as a present. The first copy of cloned software that you use for yourself, for me, is in the same boat. I know it doesn't make sense, but the millions of additional copies one can make from that 'for own use' software more or less are collateral damage)
