Exactly this, Amazon's recommendation engine doesn't have the domain knowledge about chemicals and such, what can be mixed with what. Their recommendation may mislead people.

The only results I get for "sodium nitrate" is in "Herbs, Spices & Seasonings" (at low concentrations) as part of a salt curing kit.

In fact in the UK I'm not seeing any results that could be described as a "suicide kit".

The US site does seem to stock chemicals but from a quick skim I don't seem to get any sodium nitrate in the results.

How would that even address the problem? Someone buying these things to commit suicide will still buy these things even if they need to do a few more clicks and searches because the items aren't conveniently bundled or recommended.

Yes, people who want to commit suicide might still succeed. That doesn't mean Amazon or you or I need to make it easier for them or facilitate them.

That also doesn’t mean Amazon or you or I need to make life harder for the 99%+ of the population that isn’t going to top themselves.