Seagal clearly knows his aikido. I don't think anyone questions his aikido chops. What I think many aikidoists resent is the way Seagal acts. He is, for better or for worse, the most famous aikidoist in the Western world, and so his behavior reflects on us all. Perhaps it's unfair of us to expect Seagal to be an ambassador for our art, but his celebrity has put him in that position.
You make a rather valid point, I think you are correct, people do tend to believe that Seagal is the western ambassador of Aikido, I doubt that this is Seagal's intention ?
As I have said before, I like his technique which is mostly tight and direct as opposed to uke floating on the end of a distant finger.
A friend of mine who is a writer for a popular martial arts magazine, interviewed Seagal, he said it was the most unpleasant interview he had ever been involved in, he described Seagal as the most arrogant person he had ever interviewed.
Co-author of `Positive Aikido`
Aikido Controversy - https://www.youtube.com/watch?v=FMuDqKOjnls