[sharkhawk]

Then tell the person to leave. Arrest is completely improper.

Was she arrested for not showing her ID, or was she arrested after not showing ID and then refusing to leave the bus?

[ctdonath2]

She was arrested for not showing her ID.
That's the perverse point: there was no "show ID or leave" option.

There was no articulable probable cause to demand ID.
She had no way to comply (i.e.: leave) short of complying with the baseless & unconstitutional demand, which did absolutely nothing to improve security.

She was arrested for doing nothing.

[Oztrich Boy]

Was she arrested for not showing her ID, or was she arrested after not showing ID and then refusing to leave the bus?

If she leaves the bus she must show ID: If she doesn't show ID she must leave the bus.