Specifically, 18650s are commonly used in vaping. A guy who goes by Mooch once tested a large number of 18650s he could find on the market - it turns out that almost all off-brand batteries (everything other than Samsung/LG/Sony) do not live up to their stated maximum continuous power draw specifications, never mind fake ones. Vaping often draws power at reasonably close to the stated limits, and the failure mode of these batteries is venting toxic gas and potentially exploding.

I assume there's similar issues with all large batteries.

In a nutshell... fake batteries on Amazon can harm people badly.

Far from it. If you're lucky, the 18650s will be re-badged recycled cells form failed laptop battery packs (usually one cell is completely dead but the rest still have a little life in them).

If you're not so lucky, it's from a back-alley micro-factory with no quality control and the cells can literally burst into flame (which has been the big issue with "hover boards" recently).

I don't consider AmazonBasics to be off-brand, personally. I've been using AA and AAA AmazonBasics rechargeables for about years, and they are easily the equal of the name brands I was buying (e.g., Panasonic and Sanyo, not Duracell).