Because like any organizational labeling, they encourage treating distinct ideas as a package deal.
This is perhaps clearer if I use a different example.
In general, my position on criminalizing activity is that it's something I encourage when I strongly prefer the state of the world when that activity is illegal, and not otherwise.
That bar hasn't been met on most drug use, including alcohol and nicotine, so I don't support criminalizing it. That said: I don't endorse the activity and I think in most cases the world is better if people avoid it.
So you can describe me as "pro-choice" when it comes to drug use... but you can also describe me as "anti-drug."
All of which is fine and dandy, except that if political groups start spending millions of dollars to promote the idea that being "anti-drug" includes support for criminalizing all drug use, and being "pro-choice" means encouraging my friends in their drug use, then both of those labels become problematic, since I do neither of those things.
And if those political groups become powerful enough, then even refusing those labels becomes problematic. If I say "I'm neither pro-choice nor pro-drug" in that counterfactual context, most people understand me to mean that I reject everything those two groups endorse. (They are incorrect to do so, of course, but it's foolish of me to ignore what people in fact do with language.)
At that point, if I'm going to engage in a productive conversation about what choices I endorse around drug use and drug legislation, the best thing for me to do is discard the terms altogether and talk about the underlying issues.
And if the conversation has already begun in a way that's centered around those terms, the best thing for me to do is not discuss those issues until the language environment is less distorted.
Good fleshing out of an important point..
To bring it back to the context of the abortion debate, it at first surprised me that the pro-life is also often packaged with anti-contraception. I imagine many pro-life people would not identify as being anti-contraception, but my impression is that the pro-life groups that are most vocal and most likely to affect cultural norms and policies are also anti-contraception.
For example, this 100% pro-life person claims that contraception is 100% bad:
http://stobie.home.sprynet.com/religion/100prolife.htm#contra
...Is cont
Abortion is one of the most politically-charged debates in the world today - possibly the most politically charged, though that's the subject for another thread. It's an excellent way of advertising whether you are Green or Blue. As a sceptical atheist who thinks guns should be banned and gay marriage should be legalised, I naturally take a stance against abortion. It's easy to see why: a woman's freedom is less important than another human's right to live.
Wait... that sounds off.
I really am an atheist, with good reasons to support gun bans and gay marriage. But while pondering matters today, I realised that my position on abortion was a lot more shaky than it had previously seemed. I'm not sure one way or the other whether a mother's right to make decisions that can change her life trumps the life of a human embryo or fetus. On the one hand, a fetus isn't quite a person. It has very little intelligence or personality, and no existence independent of its mother, to the point where I am comfortable using the pronoun "it" to describe one. On the other hand, as little as it is, it still represents a human life, and I consider preservation of human life a terminal goal as opposed to the intermediate goal that is personal freedom. The relative utilities are staggering: I wouldn't allow a mob of 100,000 to kill another human no matter how much they wanted to and even if their quality of life was improved (up to a point). So: verify my beliefs, LessWrong.
If possible, I'd like this thread to be not only a discussion about abortion and the banning or legalisation thereof, but also about why I didn't notice this before. For all my talk about examining my beliefs, I wasn't doing very well. I only believed verifying my beliefs was good; I wasn't doing it on any lower level.
This post can't go on the front page, for obvious reasons: it's highly inflammatory, and changing it so as not to refer to a particular example would result in one of the posts I linked to above.