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I don't use Summit but Voyager also does this and personally I find it to be useful as being told that downvoting is disabled on a particular instance tells me everything I need to know.
Instead if the mobile app itself disallows users from downvoting, now it becomes likely that users will continually complain to the dev on why downvotes are disabled even though they wern't really the ones who disabled that feature/possibility space.
It might seem silly but giving users the option to downvote even if the instance has disabled it is typically better than removing the ability to do so because it doesn't violate the principle of autonomy.
