I think it does depend though on how one looks at "oppression."
The reason I wouldn't consider it "oppression" is that there is nothing coercive about the act of displaying a particular license plate. Of course that was the same argument that advocates of prayer in school make - "we're not forcing anybody to pray or to adopt our views, so what's the big deal?"
And the big deal is something that you have hit upon - measures like these subordinate others on the basis of their own faith and conscience.
For me, the important point is that a measure does not have to be coercive or directly "oppressive" to run afoul of our Constitutional separation of church and state. This one clearly does.