why not infect .py files directly?
Btw, a nice (GUI) tool for examining/disassembling pyc files is: http://code.google.com/p/pychrysanthemum/ (it supports all the different python and bytecode versions without having to mess around with opcodes.py files...)
when was the last time you have read python files of installed 3rd party software to find viruses and trojans?
and even if you bothered, by simple obfuscation you would need to be an expert to identify a trojan in a python file.
In addition a simple way to find or clean up such a virus would be to compile .pyc files from their sources during a virus scan.
in short that can't be the reason.