Most interfaces don't allow changing settings of a feature that's turned off or disabled for whatever reason. Users can change the settings if they turn the feature on or if it becomes available. Why not let users change settings when the feature those settings affect is deactivated?
This is mostly due to the current interaction model in which either you click on the item to toggle and therefore you go through turning on the feature to take a peek inside or its access is blocked when disabled.
I can understand why in some cases being able to tailor some setting prior to turning it on it could be a plus. Like for example if I want to turn on sending my location to other devices but right before I take that big leap, I want to make sure I customize the set of people I can share that with. In scenarios like this, it makes a lot of sense and should be available for users at any time. It will help users understand and trust the system before granting it rights and/or access to privacy.
In most cases tho allowing tweaking in settings that users have decided not to use would be a time investment they get nothing from. In those cases it makes sense it isn't easily messed up. This applies to most of the advanced features 90% of users don't understand and the most likely outcome of them changing them would be an error in the experience.
I hope this helps :)
Good question. The main reason that jumps to my mind is not confusing the user. If the settings of a disabled feature are available the user might expect to change something not being aware that the feature is disabled all together an therefore the settings have no effect.
By not letting the user change settings of disabled features it makes it very clear to the user that the feature is disabled.
If there are a whole bunch of settings the user is forced to scroll an it might happen, that the switch for enabling/disabling the feature is not visible anymore.
I guess much depends on exactly what the feature is, and how intimate the end-user is with your product.
Having the ability to tweak settings for a feature that is not available will on the whole risk causing confusion - i.e a user has an expectation an action they take will impact their experience.
IF this was to be something you wanted to explore though - I would say communication and language around how you inform the user is key. You'd need to be explicit that the change they are making would NOT impact on the product at this time.