What you’re talking about now is not a “rear-ending” situation per se, but a “failure to yield” issue.
Most, if not all, highway entrance ramps have a “Yield” sign on the ramp, or it’s generally understood that merging vehicles must yield the right of way to the traffic that is already at speed on the roadway they’re attempting to merge onto.
One of the most basic rules of the road – any road – is that “overtaken vehicles”, that is, vehicles being passed, have the right of way over the passing vehicle. The passing vehicle in most road situations is expected to respect the right of way of the overtaken vehicle and only pass when safe to do so, in a manner that doesn’t impede or interfere with the passed vehicle, and leaving enough room for the overtaken vehicle to continue its travel without having to swerve, stop or otherwise take action to avoid a collision.
That’s the typical situation, but in a “merge” situation the rules change. That’s why merging vehicles are expected and required to “yield the right of way” that they might enjoy… if they were already on the roadway, which obviously they are not. It is generally understood that the merging vehicle is not yet traveling “at speed”, so for most purposes it is an “overtaken vehicle”. But since the driver of this vehicle is required to “yield right of way”, the onus is now on him to avoid the collision with other drivers. He can’t insert himself as a slower vehicle into a lane filled with faster vehicles and insufficient space for the others to react to him. So if a collision occurs here (assuming the vehicle that’s already on the roadway is exercising “normal” care and diligence, not excessively speeding, traveling with lights at night or in bad weather, etc.), then it’s more often than not going to be a “failure to yield right of way” situation, and the fault of the driver going too slowly (or making sudden lane changes) while merging.
This can also occur, for example, if a slower vehicle in an adjacent lane swerves suddenly into your lane immediately in front of you and you hit him. That’s not a typical “rear-end” situation, since there is almost no way for a reasonable driver – at any speed! – to avoid that sudden collision.