I hope this doesn't cross the abortion-debate line, but I can respond to this:
The problem applies both ways. "Pro-choice" implies that the people on the other side are against choice. Both "life" and "choice" are very powerful connotation words. No-one wants to consider herself against life - it connotes frolicking children and smiling babies and verdant pastures and what-have-you; and nobody wants to consider herself against choice, especially in a country which prides itself on freedom and individuality.
"Anti-choice" is, for one thing, a negative statement of the position (we talk about people being "anti-", as in negative, angry and bitter); for another, it connotes people who are anti-choices in general. "Anti-choice" sounds like pro-uniformity, pro-drones and clones, pro-Big Brother and mindless sheeple, anti-forty-one-flavours, anti-creativity, anti-individuality.
I am pro-life, and I call myself pro-life because the point is that I believe in the humanity of the unborn child. Yes, this thinking leads to laws which prevent women exercising one choice (not choices in general), to kill said child; but the point is not to deprive women of choice just for the heck of it, it is to protect to lives of unborn children. Similarly, you are (I'm assuming) pro-choice, and call yourself that because you believe in women's right to choose to terminate a pregnancy. That thinking leads to laws which cause the deaths of unborn children (ending those lives, but not life in general); but (again, I assume) the point is not to kill fetuses just for the heck of it, it is to protect women's rights not to carry a pregnancy to term. The name of the movement should reflect the philosophy of the movement, not its necessary consequences.
For the record, I also strongly dislike the use of the term "anti-life". I believe pro-choicers are sincerely advocating for what they believe to be the rights and welfare of women, and that they generally believe deaths of the fetuses to be a regrettable consequence, necessary for the greater good but by no means desirable. I also believe pro-lifers are sincerely advocating for what they believe to be the rights and welfare of unborn babies, and that they generally believe the distress of an unwanted pregnancy to be a regrettable consequence, necessary for the greater good but by no means desirable. There are a few extremists on both sides (the woman who had multiple abortions for an art project, people who believe women should not have access to contraceptives or higher education) who can legitimately be called "anti-life" and "anti-choice"; but very few.
And any discussion of abortion in which the other side is referred to as "anti-[positive connotation word]", in my experience, gets everyone's hackles up very quickly and results in each side demonizing the other and a flame war (or mod slap, heh).
In other words: yes, I think there's good reason for the rule. It's not just about courtesy, but about understanding the point of view of the other side.