When dividing 0 by something, the scale won't be considered as it is sure there will never be any decimal values.
You're right, your scale applies, for example try with echo "scale = 3; 1/2" | bc and the output will be .500
Also, if you use a scale only to avoid truncating, you can use the -l options which will allow 20 decimal digits.
This also prove that when dividing 0 by something, the decimal are ignored as when you type :
echo "0/2" | bc - l
the output will be 0
echo "1/2" | bc - l
the output will be .50000000000000000000
I hope I made it a little more clear.