[Avodah] Mashke of the Beis Mitb'chaya

[Sholom Simon]

In Eduyos 8:4 (and Keilim 15:6, and Pesachim 16a, and Sanhedrin 37b) 
we learn that the mashke of the Beis Mitbichayah is an exception to 
the rule that the seven liquids are mekabel tumah.

But I couldn't find any reasons.

Any thoughts as to why?

-- Sholom