Why not allow custom stylesheets? Because the vast majority of Apple's customers can't write them, is why.
As I said below, perhaps the specified fonts should be a default with alternatives in a menu, but until then, it's vastly preferable that they use the reader's font and not the book's.
This isn't a printed book, as the woeful justification should prove. Designers don't get nearly as much control.
(Ultimately, I think they will offer an option to use the specified fonts. There's so much wrong with iBooks it feels like the most unfinished 1.0 product out of Apple since Aperture)
As for putting them in the browser: a vanilla browser is possibly the worst way to display seriously large amounts of plain text. By the time you've added all the custom elements needed for good reading, you're at a full app anyway. It could be a web app, sure, but that's a different discussion.
You don't need to give the user the full power to write a custom stylesheet, just a friendly way to change common properties like paragraph font.
I am not suggesting a vanilla browser at all. I am suggesting putting all of the browsing-like functionality (books, newspapers, arbitrary hypertext) in the same application. I don't care whether it is compiled-in, a plugin, or a web app but if it's not even in the browser then it's a bad user experience from my perspective. I want a unified reading experience. I want unified search shortcuts. I want unified bookmarks and hyperlinks in books that open web pages in tabs.
Forcing users to switch apps for a data format that accomplishes most of the same things that the Web accomplishes diminishes the value of the format to the user. Application balkanization is not the answer and the fighting between vendor and publisher over page styling has already played out in the browser.
As I said below, perhaps the specified fonts should be a default with alternatives in a menu, but until then, it's vastly preferable that they use the reader's font and not the book's.
This isn't a printed book, as the woeful justification should prove. Designers don't get nearly as much control.
(Ultimately, I think they will offer an option to use the specified fonts. There's so much wrong with iBooks it feels like the most unfinished 1.0 product out of Apple since Aperture)
As for putting them in the browser: a vanilla browser is possibly the worst way to display seriously large amounts of plain text. By the time you've added all the custom elements needed for good reading, you're at a full app anyway. It could be a web app, sure, but that's a different discussion.