Why is num_classes a command line argument and not directly inferred?

[demongolem-biz]

I take num-classes to be the distinct number of relation classes which can be annotated against and predicted. With this statement, I struggle with the two following points:

1. If we are telling the training code how many classes there are, shouldn't we be telling it which classes we are using?
2. Shouldn't we be able to look at the training data and pick out how many classes there are and what they are? This would seem the most sensible solution.