The poor tram driver was probably stuck in a really difficult and (for them) unprecedented situation. There are probably rules about opening the door at incorrect spots and if the guy got out and got skittled then maybe the driver would face repercussions.

If the driver had never experienced that kind of situation before they might have panicked, and panicking people have been known to follow some internal script for how to behave that isn't necessarily rational. If certain operational procedures have been drilled into them, they might fall back on those procedures.

Or maybe the police had told the driver not to let the fine upstanding citizen out based on their own risk assessment which may not have all the information and may nor may not have been the best decision. Who knows? It's interesting to think about as a thought experiment after the fact, and of course nobody got seriously hurt (beyond some psychological injury) so it could be argued that it was a successful series of decisions. The only question is what lessons to learn so that it could be done better in future.