Why do pro lifers always say that women would use abortion as a form of birth control of it remains legal?

Because the pro-life/anti-choice stance is rooted in sexism.

Their stance is often about the idea that women shouldn't be allowed to enjoy sex or have sex freely outside of it being for the sole purpose of women becoming mothers, they like to slut-shame women who choose to have abortion or who support abortion by suggesting abortion is just about women being whores and looking for an easy way out of responsibility. Thus why those who hold this idea are often also opposed to birth control too!

Also simply put, many of those who are pro-life/anti-choice are plain out of touch with the real world, their arguments aren't based on logic or clear-thinking so they don't realize that abortion is not a form of birth control, that abortion is difficult physically as well as emotionally, and abortion is far more expensive than using birth control.

No one uses abortion as birth control - birth control refers to pregnancy prevention not pregnancy termination - some women do not use birth control (whether as they're irresponsible - sometimes outright irresponsible but this would also include those who are victims of drug addiction so not always able to be responsible, have no access to birth control, lack education on birth control, for cultural reasons, due to reproductive coercion and sexual abuse, etc.) so abortion thus becomes their only way to prevent becoming parents. That's not the same as using abortion as birth control and certainly isn't abusing abortion, those ideas are very dangerous ideas to perpetuate.

Well... they do.

well, some already use it as birth control...

since sinead thinks it is rare: http://www.mirror.co.uk/news/uk-news/thousands-wom...

Probably because it's true.