Another thing to consider with a LiPo battery option is it has to be specifically made for this device. Strictly proprietary. How long do they want to keep sourcing those out?? And how much would that affect the cost??? There will always be an abundant supply of 18650 batteries. And by the time you are ready to replace this one there are probably going to be better ones that they can put in.
Honestly, you can probably outsource LiPo batteries cheaper than 18650s thanks to them being used in several different markets. They aren't proprietary at all. In fact, there are wayyy more size/capacity/power options when it comes to LiPo.
I maintain that if you aren't going to make a battery user serviceable ... you might as well use LiPo. Devices can maintain a smaller profile while delivering more discharge power. The only logical reason I could see in NOT using LiPo would be if they've noticed a tremendous amount of bloated cells when servicing them. And I highly doubt that has been the case.