Some breeders, Mr. Nice included, think that plants from feminized seeds aren't as strong as ordinary ones, and that house is all about max quality.
As a matter of practice, creating feminized seeds from the various seed lines involves more than doubling the seed maintenance work per strain, including requiring twice the breeding space. Its not worth it unless there is a strong demand for them.
If you're buying from Mr. Nice you're probably after the best genetics possible, and if so, not only will you probably be willing to cull out a few tiny male plants, but you may even WANT them for breeding stock.
Personally, I wouldn't buy femmed seeds from that source. . .if I'm buying anything from Mr. Nice, including (and especially) his "Black Widow", I'd *want* males so I can generate more seeds or more easily cross with other plants!
Also, I believe Mr. Nice actually has offered limited runs of fem seeds on its auction site, so its certainly NOT because they don't know how to create them!