Why would anybody buy the lower capacity battery kit? Is it just too stay under 249 grams and obey the law?
Because a., the world is not made of US alone and as you noted may have other regulations (with Japan even lower mass), and subsequently b., abide by these regulations for a safer and more pleasing way to enjoy the hobby.
Though I certainly admit some rules are quite strange or very hard to understand or follow every time down to the last dot and comma. But as with most things in life, common sense and respect are your best buddies.
I am mostly worried about other countries, especially Europe. I travelled a lot with my
Mini 2 and I never had any problems with it, but I am wondering whether
Mini 3 with the bigger battery would cause some issues. I am just trying to figure out which configuration would be less problematic when travelling.
It's not only that you have to care about the mass and the category, but in EASA airspace with sub 250 g and making recordings, you are always expected from C0 down to legacy drones to at least register, do the online basic test and most and foremost have proper insurance for your drone - yes, even recreationally.
The good thing is, having this principle certificate, which you can all organise from home, you are 99 % good to fly in as many countries as the EASA has. In earlier times, there was always quite the struggle if or if not tourists are to allowed to fly for their short period of stay. Some countries had rules with benefits, some didn't, some excluded them like the CAA in UK nowadays, who can't be bothered to accept EASA certification after Brexit (yet the all the regulations are surprise, surprise nearly identical, as they have been prepared prior leaving the EU).
To sum it up, sub 250 g is the easiest way in most countries, ie to have the lowest regulation and the highest freedom of flying with some specifics you should care to read before heading to another area.
Happy flying!