[irrational]

These are private bus lines like Flixbus.

[brnt]

They require ID here in Europe. They don't always check for it, but probably they want to avoid delays in case the bus is checked at a border.

[wolverine876]

Are there public interstate bus lines?

[irrational]

Public? No. Though, the one most people think of is Greyhound and maybe they do check ID. I was just emphasizing that I was not using Greyhound.