The problem is that, in a lot of cases, there's little QC on the third-party batteries, or even necessarily ways to validate the battery, since they're all just going to be using one of a handful of "copied" chips.

Then you get into fun with who's liable if a battery explodes - many of these vendors don't really have much concrete existence, and good luck with a "warranty".

Source: tried playing hard and fast with batteries a few times, including helping someone solder his own cells into a dead first-party battery.