Hi,
well, there's not much to say to that - it is a bug of design. Right now we are in a process of this component rewrite from the scratch, in the avast version 9.0 it will be logging much less and standartly to the system log.
Currently it is possible to limit the amount of logged information by editing of /Library/Application Support/Avast/files/com.avast.daemon.plist: Set value of a parameter '-m' to 0x00000000 and reboot.
Best Regards,
Martin