Shitas Tosfos is that because the "megupah" is not a "keli" therefoe one should make it stand by itself.
Q: What is it different then the "Keli" the "Kohen" has been "mekabel" the blood after "shchita" it had a "shem keli" on it and was not able to stand by itself because it was "meshupah" at the bottom?
Yesroel Gruber, Brooklyn, NY
The logic of Tosfos is expressed explicitly by the Mishnah in Kelim (2:5).
Regarding your question regarding the receptacles used for Kabalas ha'Dam which were pointed on bottom and not able to stand by themselves, see the Mishnah in Kelim (2:8) which discusses the "Lapid," about which the BARTENURA and TIFERES YISRAEL write that any Keli which is normally used in such a manner -- even though it is pointed on bottom -- is Mekabel Tum'ah, and it is not comparable to the covering of a Chavis which is not normally used as a receptacle.
Mordecai Kornfeld