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Amazon has an image to maintain, essentially. They don't even generally allow automatic knives, most nootropics, etc. Anything that's even vaguely sketchy or could potentially become illegal very soon is typically verboten.
eBay has similar restrictions, but has a much harder time enforcing it. I've purchased and sold a number of things on eBay that their policy clearly states I should not have been able to, but at no consequence.
I don't think it's a "big pharma" kind of thing - these are largely untested and very young compounds whose long-term effects have not been fleshed out. Very few of the SARMS available are even in human clinical testing. Really, the bodybuilding community has eagerly volunteered to be a (relatively useless due to lack of control in variables) test subject.
Who knows? Some of these may be abandoned and thus will be available to us through grey-market sources for quite some time, but some - like Ostarine - will almost certainly become federally restricted at some point; sooner than later.
I am, of course, no expert about any of this. That's just what makes sense to me given what I have seen and extrapolated.
