True, sorry - It should have a popup message for phones saying it probably doesn't work. I've made zero effort to get it working on phones since you can't print from them anyway. It's just a bit of playfun really!
I frequently print from my iPhone and iPad :)
That being said I definitely saw the pop-up in Safari on iOS 8.2. I appreciate the warning, would've got frustrated without it.
Yeah, it's very annoying when stuff doesn't work on a phone. I'll be honest - I didn't actually expect anyone to print a book. I don't even own a printer so I haven't tested it!