It is an option - you can set the default to whichever you want in tour own Project Templates - the use case of -ve Cut Slopes and +ve Fill Slopes is not as common as the normal +ve Cut and -ve fill slopes, however if you are encountering these a lot you can change your default in your project Template, save the updated Template and then you have it always in that way.
Be aware that if you change that setting having already created a number of Corridors, you will need to go through all of your Templates and Change the Sign on all Tie Slopes in all Templates manually - we do not do that for you automatically (I would like to see that changed as you often dont know what is in the project until you get some way into the model). I know a few Road Guys that have changed the default behavior and now always enter the signs on all Cut and Fill slopes.
The reason it is not the Default is that we added this feature some time after Corridors was added to the software and we didn't feel that it was appropriate to change everyone overnight to the new way of doing things - this is always a challenge when we add "capability" to a function after it's first release and we have a lot of trained people who know how it used to work - not everyone reads release notes and many people would miss the change and then wonder why their slopes were going the wrong way - and that creates a huge support burden on Trimble and the Dealers etc.
Hope this is understandable
Alan