Special Teams Scoring
I think it is bad to count points via blocked punts/kicks returned for TDs or KO returns for TDs as points allowed for a defense. Those are fluky plays, generally speaking. Those fluky plays should just be rewarded to the DST who scores, but not penalized for the teams whose special teams unit gave them up.
Voodoo Chicks commented
Most don't agree with you, because most leagues use D/ST. However, in private leagues, the commish can opt out of that scoring. It's a basic D/ST setting. So Yahoo doesn't have to change anything for you to get what you want.