You missed it to some extent.
The morality and conscience of the woman wearing the shirt would not be impugned, as she has already “owned up” to the evil act.
Someone, however, that doesn’t want to be faced with their guilt, would feel accused and convicted by that message.
Okay, I misunderstood who "they" referred to.
What sort of flak could a wearer of a shirt get that would stick? The pro-abort people claim abortion is nothing to be ashamed of, so they couldn't attack the wearers of such shirts on that basis. And if they want to respond to the wearer of a shirt by saying they don't regret theirs, the wearers of the IRMA shirts could say that they're not trying to attack anyone else's decisions, but merely making a statement of personal regret about their own.
I would really like to see the wearing of "IRMA" shirts become a movement, and I would think it could happen. No overly political message, nor fire and brimstone, nor moralizing, nor anything of the sort. Just four words: "I regret my abortion". Worn to the mall, to the park, or wherever. Indeed, 1,000 people wearing such shirts as they go about their business in cities across the U.S. would probably have a bigger impact than 10,000 such people in a march.