Bicycling

They're definitely out there, but folding bikes are somewhat less standardized than full-sized bikes so it's a bit harder to guarantee that you'll have a good place to mount the batteries, run the wires, etc and still be able to fold the bike properly.

And anecdotally I see far less folding bikes and even less folding conversions around me. Maybe (probably) they're more common in London than in a semi-rural American suburb like where I live, but I suspect by overall numbers there's probably still less folding bikes (electric or otherwise) than full-sized bikes. From a numbers perspective, are you more concerned about the fire risk of say 1 folding e bike, or 100 non-fording e bikes?

Yes, most ideally it would just be a ban on unsafe electronics, but you run into an issue with enforcement. Are they going to have mandatory safety inspections, approved vendors and installers, a list of safe models, etc. and check that every e bike that passes through checks out? Sounds like kind of a nightmare to implement. Probably easier to just accept the smaller risk due to smaller numbers of folding bikes and blanket ban regular ebikes.

Also, I don't know the situation in London, but a fair amount of non-folding e bikes around me seem more like underpowered electric motorcycles with token pedals than traditional bikes, bigger, heavier, fat tires, really not well-suited to taking on the subway. If that style is common over there too, this kind of has the added advantage of keeping them out too.