True, the responsibility for safe lane change lays on the driver who is changing lanes. However, any sensible country also prohibits obstruction of a lane change, i.e. you can get fined if you don't hold enough distance to the driver in front of you to allow merging, don't make room for the person with the signal on in reasonable time or if you deliberately close the gap so the other person can't change lanes.

That kind of cooperation is mandatory for good traffic flow and properly made laws try to ensure that. Turning signal should result in people noticing you, and letting you safely switch lanes. Too often people get into some vigilante-mode because they see the other driver skipping in line or something similar.

Merging is another story. Merging traffic should explicitly yield to all traffic already on the highway, similarly to how it works in a roundabout. This prevents people merging on a highway that's over its capacity, so the traffic clears quicker. It means that traffic should queue on the ramp until it's safe to merge, indefinitely if necessary. Mathematically it makes sense, but goes against intuition.

Some municipalities have tried out metered entry on highways, that block turning onto the on-ramp altogether if the level of congestion is too high. Some trials have already ended due to the perceived injustice as well, as people already on the highway are typically from out of the city, and thus preventing those living closer from merging onto it. Personally I think if you're close enough to complain about that you should be in public transit range of the population center, and complaining more about the lack of alternatives to driving.