They can do it and they are doing it. KNC were accused of such mining in the past. I also think BFL, which was a scam on many levels, also purposely delayed shipping to mine for themselves.
This is a great way to make money if you don't care about the company's reputation, but are entering the market for a quick buck.
the knc case was different, because they were directly producing their asic i remember this, therefore they were testing it quickly after the production is complete, i see no issue with this
yes maybe they went to far with testing of some product, and delivered it later but it's different than what BFL did, which is plan scamming customers