the Maharal (Chiddushei Aggados Chullin 91a) says the Avos only kept the positive commandments but not the negative ones
No, he said it about the generations after the Avos.
For example, a pregnant woman does not need to fast on many taanaisim because of Rabbis determining, through details of the halachos of fasting, that the individual circumstances of this woman make her exempt.
That has nothing to do with the reason for the mitzvah.