Rowan
Before the changes made recently it was always an option to Set this to Yes and it was put in the software to solve a rare occurrence of Negative Cut Slopes (Yes Downward Cut Slopes do happen) or Positive Fill Slopes (upward Fills also happen) but they were not the norm.
Changing it by default is easy - just reset your Project Settings in your default projects if you want this for the future. If you change this setting mid project, be aware that all Cut / Fill Slopes need to be edited so that Fill slopes are entered as -1:1 not 1:1 as previously - this can be a pain to do (all instructions or tables that use a Cut or Fill Slope have to be edited manually as we don't change them automatically for you).
Now that we have the ability to use this feature in Subgrades, and because Subgrades can be Positive or Negative Cross slope (because they match pavement) but because the subgrade instructions are always entered as a Cut Slope (because they are below Finished grade), we now have to have this setting applied. Not everyone used Side Slopes to create Subgrades, nor does everyone know that this changed / became possible, so for now we decided to leave the default setting behavior as it was - in the training sessions that I do I tell everyone to reset this now and start to use Negative Fill Slopes and explain why, however this takes time to filter down to all users etc.
Hope that this explain the background reasoning - I agree that in a perfect world this would now be the default behavior (and I am sure we will get there)
Alan