hm, not by design. Mostly (I think mostly anyway) prices are visible, also for free agent trades...
It is probably a presentational bug.
In any case, no user can make a price Unknown. So it is not that this could be used to cover up for some prices or anything, it just happens by accident...
I'm not sure it was reported in the bugs forum yet, but I would think it has been, and I'm prety sure (even if it wasn't reported) the BBs are aware of the bug already. So I don't think any action is needed right now. ;)
They are not your friends; they dispise you. I am the only one you can count on. Trust me.