Oh god, that is the most annoying complaint I ever hear. Such a common post on any moto-focused discussion site. Usually involves an idiot on a bike cruising next to a car and then overreacting when the car begins to merge into their lane. Happens all the fucking time if you've ridden for longer than a day. Get over it and get better at anticipating it.
Two rules of riding that have kept me safe for a couple hundred thousand miles of riding:
1. Always assume the cars next to you will merge into your lane.
2. ABP, Always Be Passing. Going at the flow of traffic is dangerous. Going slower than traffic is even more dangerous.