Set $wgNamespacesWithSubpages to true (1) for NS=10 (NS_TEMPLATE) for ru.wikisource.
'+ruwikisource' => array( 10 => 0 ),
It seems to be there simply due to historical reasons (old default data, right?). The local community decided to move to the current default data for this namespace like it used in ru.wikipedia and many other wikisource sister projects.
Link to the local discussion amongst sysops: here. No voices against.