Without more identifying information about the letter you read, I cannot say whether or not I have read it, so I can limit myself only to general remarks. If this writer is a reader of an orthodox Catholic magazine though, I suspect that this person is at best being inconsistent. There likely are any number of political issues on which he or she welcomes the Pope's input, such as abortion and same-sex marriage, for example. The problem very possibly is that the Pope has taken a position on environmental issues with which this person disagrees, and so this person is trying to silence the Pope by saying he should not speak out on matters of politics. It is the same tactic pro-abortion and pro-homosexual activists use to try to silence the Pope when he speaks out against abortion and homosexuality.
In any event, the environment may be a current political football, but concern for the environment is not merely a matter of politics. Right stewardship of God's creation is both a religious concern and a moral issue. As such, Pope Benedict XVI and other Church leaders are well within the parameters of their "job descriptions" to call for care for the environment and for all of God's creatures. Specific solutions may require political negotiations, which is primarily within the competence of laity; but input from religious leaders, Catholic and non-Catholic alike, should not be excluded from consideration.
It should also be said that to drive a wedge between "spiritual pronouncements" and the material world, which is what God's creation is, treads very close to the heresy of Manichaeism.
Is environmentalism a moral issue?