Different countries have different customs, ways of life you could say I suppose. To live in the country and be accepted you have to adopt other systems. Switzerland is no exception. I was quite green when I arrived in Switzerland, grew up in England and actually was of the opinion that all countries were the same, they just spoke different languages.
Here you see empty seats at a roadside café in our local town. Of course you can sit down and order something to drink, no problem. It is all empty, but what do you do when one seat is occupied. In England you sit on an empty seat. Perhaps I am overdoing it a bit, but you never know if the person occupying the one seat, is saving some seats for colleagues or family. Everything is possible. You ask politely, “is this seat free” pointing to the one you want. Generally the answer is positive and you sit with no problem.
On a train the case is more complicated. More seats are occupied, but never ever just sit on an empty place. Ask the person sitting next to the empty sear “is this seat free?” It would be considered impolite not to ask, almost a criminal offence. You never know. Perhaps the other person is on the toilet, looking out of the window in the corridor or not yet sitting. Accommodation can be an unknown problem in Switzerland. Not to mention if you enter a restaurant and sit down. It might be that someone tells you, quite decisively, this seat is occupied. So remember, there is an unwritten law in Switzerland, ask first. I have learned this the hard way.