But a sibling comment to yours by shoyer argues that mutable objects can be elements of a tuple. A tuple whose elements are mutable is effectively mutable.
Can (1, 2, [3]) be used in a hash table, and what if the [3] is mutated to [3, 4]?
When it tries to hash the tuple, it finds the list inside of it, realizes the tuple is mutable, and throws an error.