According to the standard specifications the scheme attribute seems optional, this is probably the main reason why so many publishers omit it.
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The identifier element has an optional OPF scheme attribute defined by this specification. The scheme attribute names the system or authority that generated or assigned the text contained within the identifier element, for example "ISBN" or "DOI." The values of the scheme attribute are case sensitive only when the particular scheme requires it.
This specification does not standardize or endorse any particular publication identifier scheme. Specific uses of URLs or ISBNs are not yet addressed by this specification. Identifier schemes are not currently defined by Dublin Core"