I recently traveled with FlixBus through France. Both coming from the Netherlands and from Spain, the bus was stopped and checked by customs. Coming from the Netherlands, baggage was checked (I suspect for drugs) and on the way from Spain only ID cards, visas, etc. were checked.
I'm confused why this is. I figured that because no checks are carried out at the borders between Schengen Member States this wouldn't happen.
Why are these checks done?