The reasoning for this is for consistency, while still maintaining autocomplete proximity.
(Also, the comments in the header file and the README file contain the name "capitalizedStringInPlace" instead of "capitalizeInPlace")
Another inconsistency is that most mutating methods in ObjectiveC don't return self, but either void or a boolean or an integer that reports if the method succeeded. But I assume that this inconsistency is on purpose because you want to make the methods chainable.
I don't disagree with you though. I'd prefer if the interface for this was more idiomatic.
Returning void is sort of pointless. A boolean, maybe. I feel like returning self is just making better use of things, really, and probably doesn't really cause any confusion.
Returning void makes it easier to spot errors. If you accidentally use "capitalizeInPlace" instead of "capitalizedString" your code will compile without issues. If the mutating method returned void, it would result in a compile error.
a) The author might disagree with my asessment. A pull request would imply that I consider it an error, but I think it's more a question of style.
b) I already waste too much time on HN. Forking a project, trying to understand all of it, making changes, writing a pull request, etc. would probably take me all day but I need to clean the kitchen and take care of the kids and work on some overdue client work that I have been postponing for far too long.
Take for example the capitalize methods. In ruby, you have "capitalize" and "capitalize!" to distinguish the immutable and mutable versions. To make this distinction in ObjC, you generally use nouns vs. verbs. So for the equivalent of ruby's "capitalize", you'd use "capitalizedString" (which is in fact provided by NSString). But the mutable version, the equivalent of ruby's "capitalize" should simply be "capitalize", not "capitalizedStringInPlace".