There's no excuse for being rude, but there can be a good reason for sitting in the folding seats.
Our local trains have two sets of folding seats (officially space for a wheelchair) and I used to often sit on those seats when I commuted by train I had lower back pain on and off for a few years and this was the only way I could commute to work - I couldn't stand for long, use the 'normal' slouchy low seats on the train (more specifically I couldn't get up from them), drive or use the bus. If they were all taken when I got on the train, I had to get off and wait for the next train.
If it hadn't of been for the folding seats I woudn't have been able to work for months at a stretch.