Why a production rule $S\to ε$ has been included in the definition of regular grammar? In that case, why it is restricted for $S$ (start symbol) to be present on right hand side of any production rule?
This production rule allows the grammar to generate the empty string. We restrict it to the start symbol so that it can fulfill its function but not interfere with the properties of the grammar in any other way.
In short, it's a hack to allow regular grammars to generate the empty string.