All the inexpensive ceramic filters typically used for the 10.7MHz IF are designed to be a 330Ω impedance (www.mouser.com - Murata-Electronics - SFECV10M7GA00-RO). Matching that impedance for the output of the mixer and the input to the log-amps allows for the highest power transfer through the system. Using a 50Ω input would cause multiple issues (smaller relative signal amplitude, reflections from impedance mismatch, etc).