1. I have not found anyone who asks this question explicity, but I saw that the Hebrew Schotenstein edition cites the Chidushei ha'Gra here (DH AR'I) who writes that the reason that the Gemara here cites the way Yitzchak was bound up as an example of "Ragul" is that Chazal always want to mention the Akeidah of Yitzchak whenever there is any opportunity of doing so.

2. The Vilna Ga'on seems to be saying that if we merely would have the technical necessity of knowing what the Mishnah means when it says "Ragul," the Akeidas Yitzchak would not be the only way to describe this. However, there is a deeper reason why the Gemara mentions the Akeidah; it wants to remind us of the merit of this great act, the reward of which Klal Yisrael benefits from until this day.

3. If we take this a little further we can now understand why Avraham is also mentioned. The Gemara wants to stress not only the great Mesirus Nefesh of Yitzchak Avinu -- that he eagerly agreed to be sacrificed to Hash-m -- but also the Zechus of Avraham Avinu who was prepared to lose his beloved son. When the Gemara writes "Yitzchak ben Avraham," it is hinting that not only was Yitchak bound up in this way, but it was also Avraham who willingly tied him up and prepared him to be sacrificed to Hash-m.

Kol Tuv,

Dovid Bloom