Your example is not really the best one.
EC84 has no compulsory reservation. Only a compulsory fee for Interrail/Eurail users.
When the train is full, people without reservations have to leave for security reasons.
I stand corrected re reservation.
However that does not change the event. These 2 travellers had global passes and the inspector clearly found them invalid, possibly for not prepaying the tariff.
Pay or get off was his stance and they were escorted off at the station we happened to be stopped at.
So all those who board trains without correct documents face the real risk of pay or get off.
And where’s the surprise in this? If you don’t have a valid train ticket, then the staff enforces it like this...