Technically no. According to NATO's founding document, the Washington Treaty:
The Parties to this Treaty reaffirm their faith in the purposes and
principles of the Charter of the United Nations and their desire to
live in peace with all peoples and all governments. They are
determined to safeguard the freedom, common heritage and civilisation
of their peoples, founded on the principles of democracy, individual
liberty and the rule of law.
The treaty is generally ambiguous, however, given the above:
"founded on the principles of democracy, individual liberty and rule of law"
Article 1 begins to clarify this:
and to refrain in their international relations from the threat or use
of force in any manner inconsistent with the purposes of the United
Article 10 further elaborates on why one might join, and the standards expected of members:
The Parties may, by unanimous agreement, invite any other European
State in a position to further the principles of this Treaty and to
contribute to the security of the North Atlantic area to accede to
One could quite easily argue that Turkey is violating rather than furthering the principles of the treaty. The treaty permits members to leave, but does not discuss expulsion. What is clear in articles 10 and 11 is that the USA is the gatekeeper, and new members are accepted only at its behest. Presumably America also has the power to expel members since it is an American club.
Greece is a good counter-example, but it was an exception rather than a norm. When it joined in 1952 it was a constitutional monarchy, and when the junta came to power in 1967 it was also the height of the cold war... and so somewhat of a bad time to start kicking out NATO members.
Since the fall of the USSR fledgling democracies in Eastern Europe have opted to join for protection, and generally the whole bloc has become more democratic and free over time. Turkey's military juntas historically also claimed they were doing what they were doing to enforce secularism, democracy, and stability; which generally they did. But now it's different.