They don't, @Ever.
'Pro-choice' means precisely that -- that they believe women should have a choice about whether to terminate or continue a pregnancy.
'Pro-abortion' is a term I have only ever heard used by Christian fundamentalists who appear to think that people who are in favour of legal abortion being available are wildly enthusiastic about the procedure, and have not grasped the basic principle that many pro-choice women would not have an abortion themselves, but regardless of their own individual opinion, want the choice to be available to other women who may think differently.
That is the essence of the pro-choice position -- that it is irrelevant what you personally think of abortion, whether because of your religious views or some other conviction, because the choice should be available.
This is why the referendum legalising abortion passed in Ireland last year. Large numbers of people who did not themselves approve of abortion, including devoutly Catholic older people who had believed since the cradle that abortion was a serious sin, acknowledged that the choice should nonetheless be available to those who felt differently.
'Pro-life', on the other hand, is a vaguely and nastily euphemistic term suggesting that those with these views are fuzzy, gentle huggy types, rather than those who actively oppose other people exercising choice over what happens in their own bodies.