They can sell it because they probably aren’t making any specific health claims about it. As a food supplement, it doesn’t have to be reviewed as a drug would.
Also, they can sell it because people clearly buy it. Put them out of their misery and return it.
Why would I think you’re being snarky? Certainly, I’ve never had an objection to being asked for a cite. In this case, in fact, no I do not–unless you count a dim memory of being told this by someone as a cite. No? Didn’t think so. I haven’t been able to find anything with hard numbers regarding the population of obligate anaerobes in comparison with other types, but I have found many which do state the gut houses many varieties of obligate anaerobes. At this point, I’m perfectly willing to defer to your specialized knowledge in this area.