On erev Yom Kippur Rav Eliashiv, shlit"a, announced that Crocs are not suitable footwear for the Day o' Atonement, shocking and annoying many people who until then had used them as their preferred Tisha B'av/Yom Kippur footwear. Being Dati Leumi I decided to research what the leading posek for the community, Rav Shlomo Aviner, shlit"a, had to say on the subject. Unfortunately his decision was not that much different than Rav Eliashiv's. Although his answer was based on a shailoh about Tisha B'Av, it could very easily be extended to Yom Kippur since the ban on leather shoes applies to both days, albeit for different reasons (mourning vs inui)
However, while discussing this with a Rav that I learn with, a different thought occured to me.
According to halacha, most medication is forbidden for use on Shabbos. The gemara derives this from how medications were prepared in those times, usually through the grinding of herbs to prepare the desired therapeutics. Schikas samemanim is a derivative of tochen, hence medications were forbidden on Shabbos for the non-seriously ill because of the fear that one would come to grind the ingredients to ensure an ample supply. Even though we no longer prepare medications in this fashion, the original rule has remained in place.
The Nishmat Avraham contrasts this with another rule which is no longer in force. In the times of the gemara, there was a law that any water left uncovered overnight must be disposed of. The fear was that a snake would have drunk from it and left some of its venom in the water leading to a danger to the person drinking the water. By the time of the Shulchan Aruch, however, this rule was observe mostly in the breach because, as the poskim explained, we don't really find snakes around like in the times of Chazal.
On the surface, these two rules would appear to contradict each other. In one case the original rule remains in force even though the circumstances that led to it no longer apply. In the second case, the change in circumstances is the reason the rule is obsolete.
However, on further analysis, the Nishmat Avraham notes a major difference between the two rules. In the case of snakes and water, there is no real rule that water left overnight cannot be ingested. Nor is it usual from snakes to slither by and drink from cups sitting out. Therefore once the circumstances changed, the law no longer had to be observed.
However, medications are still produced from a grinding process. Yes, they're produced en masse without regard to how much acetaminophen I might pop this Shabbos after the rabbi's sermon but the original decree still has some relevance. Further, the Nishmat Avraham notes that many naturopaths and homeopaths still do produce medicine by grinding ingredients. Therefore even though the majority of medications are not made on demand, the rule still applies.
What does this have to do with Crocs?
One must remember that when Chazal made a g'zeirah, it was made under specific circumstances and for specific reasons. As a result, it is usually not correct to extend the decree to areas not covered by Chazal. We see this in the gemara which usually rejects such attempts as a g'zeirah to a g'zeirah which, except in certain circumstances, does not go through. For example, in some circumstances one can engage in amirah l'akum when the issur to be performed is only d'rabannon.
So let's look at Crocs. The reason for the prohibition on leather shoes on Rosh HaShanah is because the Torah asks us to engage in inui, afflication, on Yom Kippur. Chazal decreed five types of inui, the wearing of leather shoes being amongst them. They did not decree that comfy shoes are forbidden and therefore leather footwear, being comfortable, is assur. They specifically decreed on leather shoes.
(Frankly, the most uncomfortable shoes I own are my wedding shoes which are fancy leather with leather soles and all, but I digress)
So here's what occured to me. If Ravs Eliashiv and Aviner are going to darshan out the reason for Chazal's g'zeirah and use that reasoning to extend it - leather shoes are comfortable, leather shoes are forbidden, therefore comfortable shoes are forbidden - into a stricture position, then what's to stop others from using the exact same process and extending things into a lenient position. To wit: medications were forbidden because of grinding individual doses, commercial medications are not ground in individual doses, therefore there should be no prohibition on commercial medications!
I am certainly not paskening that one can now use medications freely on Shabbos in opposition to the opinion of all major halachic decisors but I would want to know: if Crocs really are forbidden on Tisha B'Av and Yom Kippur, what's the reason we can't go the other way?