I am new to this so, I don't know all the procedures in these AdCom meetings but did they know in advance who would be on the panel?
If they knew in advance who would be on the panel, you are right. They should have objected - if procedure allowed for that. But, the FDA has not been exactly forthcoming with communication to Amarin regarding how they were going to proceed during that AdCom. This may have just been another factor thrown out as a surprise. And, it would have looked very bad for Amarin if they had protested on the day. They probably knew they were screwed the moment they saw Hiatt and just hoped he would retain professional objectivity. :(