And this is what I was referring to earlier in the thread and why the non-profit status of churches makes churches slave to the state. There is not a clear distinction between politics and religion for Christians. Most Christians believe abortion is wrong through their reading and interpretation of scripture. Most people also see abortion as a political issue. So, when a pastor makes a point in his sermon about abortion being wrong, does that give the state the right to yank the non-profit status of the church? It absolutely shouldn't but some will try to make the case that it should (because churches shouldn't be making political statements!).
I whole heartedly agree, but if the article from HuffPo is accurate, and the pastors targeted by the subpoena were actively coaching their church members in filing out petitions regarding a specific piece of legislation, that has officially hopped over from saying that homosexuality is against God's law, into actively trying to influence politics. I guess my point is that while some things spoken about in church, abortion is bad, stealing is bad, or homosexuality is bad do tend to touch political elements, preaching belief according to one's belief system should still be fair game for preachers far and wide under the existing tax rats nest of laws. However unless there is a new version of the KJV that shows Jesus doesn't like the Houston Equal Rights Ordinance, any mention of specific legislation should easily be construed as mingling politics and religion which currently is a no no.
More specifically, to your example, I think preaching that abortion is wrong has been shown to be perfectly acceptable in a legal sense ever since Roe v. Wade. However, if a preacher were to get into the pulpit and say "Vote for so and so because they are anti-abortion" would definitely be intermingling the religious views of abortion and political activism. Heck, religious organizations even go as far as preaching against abortion and providing information about candidates explicitly detailing their stances on various religious hot button topics. None of that has been seen as political activism in the past, and as long as its presented in an informational way, I don't see it happening in the future.
I should caveat all of this by saying that I am uncomfortable with this not just on a religious vs. political debate, but also on a government witch hunt type level. As of now, an LGBT mayor, with an obvious axe to grind, has gone in a public medium and alleged that a church did something. This smacks of someone trying to publicly shame an organization without due process. This has especially troubling tones. The mayor has no place going into a public medium attempting to drum up support, all the while using government resources in what could be a witch hunt. On the other hand, if these church leaders did in fact use their position to attempt to influence politics directly, instead of indirectly by preaching the standard message, there are definitely some issues that need to be raised.