Seems pretty reasonable. Since bikes don't cause as much damage/injury as motorized vehicles, it makes sense to have a lower classification and lower fines as well --while keeping them responsible.
However Japan is not such a place and its laws reflect its culture.
To note, at no point you're barred from riding a bike or penalized in a critical way, so why not ?
There's little to no traffic infra in residential blocks, and you're expected to be careful and watch your way.
Also bikes can easily be 30kg heavy and motor assisted, as you're using it for groceries or moving two kids to) from school everyday.
Combine thanig an aging population (small kids freely roaming the neighborhood can also be an issue, but let's assume we're at night, not drunk at lunchtime)
Hitting someone at a corner with a bike can be fatal (and actually is, at some non negligeable rate), and hitting a car won't be a small bump.
PS: > so punishment should reflect that.
DUI in Japan is one of the highest infraction. There's a point system, and you're (temporarily) barred from using your car at 6 points. Straight DUI with no casualties, no accident, is 25 points.
The blue ticket you just have to pay that is discussed for bikes is peanuts.
You're potentially a danger to yourself and the people around you. Why wouldn't it be handled appropriately?
Getting a DUI for riding a bike while (somewhat) intoxicated does indeed sound strange from a US perspective. It's not something that generally happens or is on anyone's mind.