Views on taxes and views on abortion just not the same
Paragraph 3 of the Doctrinal Note on some questions regarding the participation of Catholics in political life:
...the legitimate freedom of Catholic citizens to choose among the various political opinions that are compatible with faith and the natural moral law, and to select, according to their own criteria, what best corresponds to the needs of the common good.
This seems to fit the idea that allows different 'opinions' on taxes and tax systems and to meet the 'needs of the common good' rather than only one way (i.e. higher taxes on the rich).
It is not the Church’s task to set forth specific political solutions – and even less to propose a single solution as the acceptable one – to temporal questions that God has left to the free and responsible judgment of each person.
Thus, to advocate for higher taxes on the rich seems an inappropriate on behalf of the Church or Iowa bishops.
It is, however, the Church’s right and duty to provide a moral judgment on temporal matters when this is required by faith or the moral law.
This statement shows the Church's duty to speak up about the evil of abortion, euthansia, SSM, etc., but not issues like taxes.
...there can generally be a plurality of political parties in which Catholics may exercise – especially through legislative assemblies – their right and duty to contribute to the public life of their country. This arises because of the contingent nature of certain choices regarding the ordering of society, the variety of strategies available for accomplishing or guaranteeing the same fundamental value, the possibility of different interpretations of the basic principles of political theory, and the technical complexity of many political problems. It should not be confused, however, with an ambiguous pluralism in the choice of moral principles or essential values.
Again, Catholics' varied 'opinions', 'choices', 'strategies', and 'interpretations' on the issues of the day are more than acceptable as long as we are talking about 'moral principles or essential values' (i.e. abortion, SSM, etc.)