It's a good idea to have some objects isolated in our portfolios.
That being said some times when we want to isolate an object that by nature is already white it does not make sense to use a white background.
The main reason to isolate is so it can be removed and used elsewhere. So if you use black it easier to do it on a white subject.
Of course any other color than white can still be done using the white background.
The whole point of this is that yes there are rules and guidelines that should and need to be followed.
But it makes sense to stray from them for common sense or just to do it different some times.
It's kind of like saying a subject should never be in the middle of the photo except when it needs to be in the middle.