Because it's a very simple concept. Sport or recreational fishing is based on capacity. When the capacity with very basic gear is exceeded they impose limits. Once your capacity is greater than what would reasonably considered a "personal" amount they consider you a commercial fisherman. This is regardless of whether or not you sell the catch.
This is a reasonable way of dismissing the claim that these "traditional" hunts are the same thing as what the Japanese are doing with these "research" vessels. There's sport fishing and there's commercial fishing.
I'm not acting as if it's the same everywhere, in fact I clearly stated otherwise up thread. Why lie about it?