When someone makes a judgment that those who don't want to participate are "high and mighty" and all sorts of other insulting descriptions, then that is out of line. If someone doesn't want to participate, skip them and go on. To get all snippy about those who choose to take a pass takes this to a whole different level and detracts from the cause of raising money for ALS. This challenge is supposed to be a fun way to raise money for ALS, as well as bring attention to it. Once you start making negative remarks about those who do not wish to participate, it takes away from the fun, causes hard feelings and the good that was being done will diminish.
If someone thinks the challenge is stupid or beneath them, ignore it and go on. Someone else will happily participate. It's about ALS, not whether or not someone wants a bucket of ice water dumped on them.
For whatever reason, some people will not want to participate. Respect that and move on. They owe no explanation and to demand one will accomplish nothing, save to irritate them even more. If they volunteer an explanation and you think it isn't valid, then really, you are the one with the issue. If they are gritchy and say they are above dumping water on themselves, just go on to the next person.
To make this about whether or not someone will dump ice water on themselves and whether their reason for refusing to do so is "good enough" will suck the momentum out of fundraising for ALS and worse, some people will have a negative association when asked to donate to funding. If it's a challenge, some people will accept and some will decline. It's a challenge, not a mandate.