Not sure if it’s going to really be a Framework-specific part, as in, that you can’t use your own battery if you really want to.
Probably a few factors made them package the bare li-ion cell into a plug-in module.
• This is a small li-ion battery. They just don’t do “standard” sizes. Certainly not like cylindrical alkaline batteries, like AAA size. A small 80mAh li-ion could be something like 401030. Which breaks down to 4.0mm thick, 10mm wide, 30mm long. There are essentially infinite sizes. And as long as you are buying enough, there is the option for you to dictate the exact size, and the manufacturer will make it. Major mobile phone companies do this.
• Framework really endeavored to make replacing parts as dead-simple as possible. The typical connectors used inside densely packed devices are tiny, and often fairly delicate where anyone not used to such connectors can easily break latching mechanisms or worse, bend pins shorting them together. There have been a number of posts by users who broke connectors in different ways, or bent pins and came for advice on unbending the tiny needle-like pins. And if it’s the mainboard-side connector, opps, there goes your mainboard! Do not pass “Go”, do not collect $200, go directly to paying hundreds on a new mainboard.
• This is a small li-ion battery, for a device intended for kids. You do not want them handling the raw li-ion cell. Short it, bend it, damage the foil package, and it can go into “thermal runaway” leading to it “venting with flame”. Aka the body of the cell heating up enough to burn you, and flammable gasses generated resulting in a pressurized stream being expelled, which then ignites (shoots a stream of flames). Oh yeah, and the gasses and smoke are highly toxic. Not only do you want to be nowhere near it, but such smoke leaves a toxic residue on everything it touches.
For those that have experience in dealing with raw li-ion cells safely, I imagine we can pry open the plug-in module to access the cell. Half hope the shell isn’t sonically welded, and half hope it is. You can usually crack the welds, but you sometimes deform the shell a little in the process, so that it no longer looks perfect.