Because they made ffmpeg to much more and do support it, asking for money as in "commercial" is completely legal.
Notice, they do not stole any code or are hurting ffmpeg.
The sane thing they should do is - work for or donate some % ffmpeg.
If ffmpeg would be BSD licensed, they would be capable to steal whole codebase and integrate it inside in such a way, that any further cooperation will be non-existant.
This way, ffmpeg would really miss the improvements coming from commercial vector. And they would do that (stealing), because they would have much more control over source, as well as completely no legal resposibility. This is BSDM loose/win.
They can do that with GPL(fork), but the source would need to stay open. Win/win.