You're probably correct that the right's anti-science views come into play. This is a party that seems to invent facts about how women's bodies work to suit their beliefs, e.g. Todd Akin. However, you have to wonder why conservatives took up that myth about emergency contraception. Did they just get uniquely bad information, or did the hear what they wanted to hear?
The science indeed has never indicated that emergency contraception inhibits implantation. If it does have an small impact on implantation, it would likely be less than ibuprofen. Since Republicans have never raised the idea of banning ibuprofen, it's pretty suspicious to me, especially since emergency contraception prevents abortion. Conservatives should be huge fans of emergency contraception and they would be if they were rational people who wanted to reduce the number of abortions.
One of the main reasons why a lot of conservatives think emergency contraceptives prevent implantation rather than ovulation is because they get Plan B mixed up with
RU-486, which is an abortifacient. There was also one study that came out in the 70s or 80s suggesting one of the birth control meds (I forget which one) prevented implantation. That study has since been shown incorrect.
Some combination of these is likely the cause. Most people don't know these things so it's no surprised the details of exactly what it is that's an abortifacient became a little confused.
