The difference between Seagal and "another 7th Dan Aikido instructor" is that Seagal made a choice and actively sought to be a star in movies, to become a public figure. How many other "7th Dan Aikido instructors" have sought out or done this.
The people criticizing him paid to see some of his movies and thus Seagal benefited monetarily from them and because of this and his status as a public figure he is a fair target of criticism by these people. Whereas" another 7th Dan Aikido instructor" not being a movie star and public figure would not be.
Let's not confuse the man and the movie. When under contract with a studio ones soul tends to get sucked out a bit.