Well, I had a look on the exhibitor site but other than a brief summary, I couldn't find any information about the group ... maybe you have a link to their website?
My experience at such meetings (not related to climatology or meteorology though) is that you always have a few 'outsider' groups who show up. In my area that tends to be the anti-vaccination groups, and also anti-abortion pro-lifers. I say 'outsider' here, just to clarify, in the sense that they're lobby groups rather than companies exhibiting products or researchers presenting studies.
In any case, I presume the people wanting to exhibit at the AMS meeting have to pay a registration fee so the organisers probably don't really care once they get the attendee money. And it's good to have a mixed bag of exhibitors because it makes things more interesting.
The bottom-line though, like everything else, is that no one is forced to listen to them so there's probably no harm in taking their money knowing the majority attending will ignore them.