She also talked about a specific junction, where people cross the stop line on green, then slam the brakes on when the lights change, when they should carry on across the junction.
In that instance, surely she is right that they shouldn't slam the brakes on?
But I absolutely agree that she is wrong to expect people to cross the stop line when the light has gone red.
It is a problem, though, that there are drivers out there who will chance it, and nip across the lights on amber or red. I see plenty of them when I am out and about in the car - driving in two lanes of traffic, I stop at the traffic lights (because I saw them go amber and slowed down to stop at them, as I believe the Highway Code says I should), and in the lane next to me, a car or cars who were going at a similar speed to me, and are in similar density of traffic, and who had the same opportunity as me to observe the lights going amber, sail through the red light!
Maybe the OP sees plenty of people doing this. Maybe she does it herself. And maybe this is what makes her think that everyone is going to do it. If so, she is wrong, and should be assuming that the drivers around her intend to obey the rules of the road. That's what I do. And if me obeying the rules of the road pisses other, less law-abiding drivers off, that's not my problem!