> It is certainly reasonable to expect "parseInt" to parse an integer out of a string in base-10 by default

It does.

> and entirely unreasonable to expect to be required to provide a base as anything except an optional argument

It is optional.

> and certainly it is unreasonable to expect that that second optional argument is treated as not optional in a composition operation

I don't understand what you're saying here. It's never treated as required, it's just map supplies a parameter in that position, so it get's used. That's how optional parameters work.

The wat (if there is one) is that map provides extra arguments.