The Kesef Mishna (Shechita 1:3) writes:
וא"ת סוף סוף נימא דבעו אסיפה בעודם חיים. וי"ל שאילו היה אפשר לומר כן אה"נ אבל כיון דא"א לאכלם בלא אסיפה שאפילו קלטן בפיו ולא אספם בידו מ"מ בעת שהם בפיו נאספים הם אם כן אינו דבר מיוחד דמזהר עליה קרא
And if you want to say that in the end of the day we should say that it requires gathering while it is alive, one can answer the following: If it were possible to say such a thing then we would, but it is impossible to eat them without gathering. Even if one grabs them in one's mouth and didn't gather them with one's hands nevertheless while they are in one's mouth they are gathered. Therefore it isn't a distinct thing that the Torah would prohibit. (my translation)
There is more to say on the matter and you can find an extensive treatment in R Yerucham Fishel Perlow's commentary to Saadya Gaon's Sefer HaMitzvot, volume 3, Milu'im #8.