Because I'm pretty sure that it's possible to use mutable objects as keys in general. The users just have to observe restrictions: like don't insert the keys into the dictionary and then mutate them while they are in the dictionary and expect the dictionary to still work fine.
(Objects can also be used as keys even if they are mutated while in the dictionary, if identity-based equality is used for the dictionary; i.e. two keys are equal if they are acually one and the same object, and not just two similar objects.)
Because I'm pretty sure that it's possible to use mutable objects as keys in general. The users just have to observe restrictions: like don't insert the keys into the dictionary and then mutate them while they are in the dictionary and expect the dictionary to still work fine.
(Objects can also be used as keys even if they are mutated while in the dictionary, if identity-based equality is used for the dictionary; i.e. two keys are equal if they are acually one and the same object, and not just two similar objects.)