If you create a closed-source derivative of bsd-licenced software AND DISTRIBUTE IT, the derivative is not as free as the original. Users of the derivative are being deprived of freedoms that existed in the original. They dont have the ability to modify the original parts of the derivative once its binary only.
And be honest with yourself, if you create a closed source derivative of BSD licensed software would you be doing it mostly for the benefit of yourself or for others ?
Absolute freedom is a fantasy, absolute freedom permits people to take away other peoples freedom (to have power over them), once that happens freedom is no longer absolute.
The distributed binary is just as free as the original binary. And they DO have the EXACT SAME freedom I have - to develop their own derivative from the same source I used. Not my problem if they don't have the talent - they can always hire someone else to do it. What they don't have is the right to tell me what I have to do.
If I create a closed-source bsd-licensed derivative, OR my own completely original or derivative BSD-licensed code, that is MY choice. The people who have hired me to work on software, either as an employee or as a contractor, didn't give out the source code, and that is why they could afford to pay me. Same with almost everyone who works in the software biz. Open source is not a obligatory, nor is it desirable in many cases (like when you want to eat).
However, closed source does NOT limit other people's freedoms - they are, as I pointed out, free to use the same original source to make their own derivative. And they're free to release the source or not when they distribute binaries, as they wish, unlike the GPL.
There is nothing preventing users of the derivative to develop their own version using their own skills to create their own derivative from the same source I use, so users have the same freedoms I have. If they don't have the skills, sucks to be them but I don't owe it to them to give them my source. They have other options, such as paying someone else to develop their own derivative.