Which OECD rule(s) do citizenship-based taxations contradict?

Question

I read on 20minutes.fr (mirror) that some French politicians are trying to pass a law exempting dual US+French citizens living in France from US taxes:

« Le problème principal, c’est que la fiscalité américaine est basée sur la nationalité, en contradiction avec les règles de l’OCDE, estime l’eurodéputée française Virginie Rozière. L’Erythrée a voulu faire pareil et a été sanctionnée. Il n’y a pas de raisons de traiter les États-Unis différemment. Ils ne se comportent plus comme un grand frère bienveillant mais comme un acteur sans foi ni loi qui cherche le rapport de force. Si on n’est pas capable de répondre, on se fera écraser ».

Translation (based on Google Translate):

"The main problem is that the US tax is based on nationality, in contradiction with OECD rules," said French MEP Virginie Rozière. "Eritrea wanted to do the same and was sanctioned. There is no reason to treat the United States differently. They no longer behave like a benevolent big brother, but as a faithless, lawless actor who pursues relations based on force. If we cannot respond, we will be crushed."

Which OECD rule(s) do citizenship-based taxations contradict?

Answer 1

The Model Tax Convention

The Model Tax Convention is the basis for agreements among OECD members regarding international tax agreements. It basically helps them coordinate taxation policy.

One of the core purposes of the Convention is to keep a person from being taxed twice for the same thing:

International juridicial double taxation can be generally defined as the imposition of comparable txes in two (or more) States on the same taxpayer in respect of the same subject matter and for identical periods. Its harmful effects on the exchange of goods and services and movements of capital, technology and persons are so well known that it is scarcely necessary to stress the importance of removing the obstacles that double taxation presents to the development of economic relations between countries [Source]

Taxation based on nationality allows a person to be double-taxed. For example, a United States citizen working in Germany is theoretically subject to two income taxes. Article 21 of the Convention says that private income should be taxed where it was earned, regardless of the nationality of the worker. In our theoretical case that means it should be taxed by Germany and not the United States. However, since the United States taxes based on nationality (citizenship) this person could effectively pay two income taxes, unless the U.S. and Germany have ironed this out*.

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/* I don't know if the United States and Germany have any kind of tax treaty that stipulates how this works. This is a purely theoretical situation used to illustrate the Convention's terms and is not otherwise based in reality.