Ironically, the USA is the one country that refuses to accept other countries' issued passports with extra pages added, but is the country most guilty of adding extra pages themselves. Extra pages are a nightmare for document security as even the authentic ones are very rarely added properly and thus open to huge abuse. "Specialised" visas (eg. work visas, etc... as opposed to regular business/tourist visas) and residence permits added on extra pages will often be subject to huge amounts of additional scrutiny.